Tests are dependent on local timezone settings

I was running the tests locally by request of someone in the #Node.js IRC channel and noticed that I got a different set of test failures depending on the timezone settings of my computer.

With timezone set to GMT (my normal timezone):

```
1) dateformat([now], [mask]) should format `longTime` mask:
AssertionError: '1:19:44 PM GMT' === '1:19:44 PM UTC'
at Context. (/Users/lenny/dev/node-dateformat/test/test_formats.js:64:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)

2) dateformat([now], [mask]) should format `expiresHeaderFormat` mask:
AssertionError: 'Wed, 26 Nov 2014 13:19:44 GMT' === 'Wed, 26 Nov 2014 13:19:44 UTC'
at Context. (/Users/lenny/dev/node-dateformat/test/test_formats.js:64:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)
```

With my timezone set to EST:

```
1) dayOfWeek should correctly format the timezone part:
AssertionError: '7' === '1'
at Context. (/Users/lenny/dev/node-dateformat/test/test_dayofweek.js:11:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)

2) dateformat([now], [mask]) should format `longTime` mask:
AssertionError: '1:19:44 PM EST' === '1:19:44 PM GMT--500'
at Context. (/Users/lenny/dev/node-dateformat/test/test_formats.js:64:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)

3) dateformat([now], [mask]) should format `isoDateTime` mask:
AssertionError: '2014-11-26T13:19:44-0500' === '2014-11-26T13:19:44--500'
at Context. (/Users/lenny/dev/node-dateformat/test/test_formats.js:64:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)

4) dateformat([now], [mask]) should format `expiresHeaderFormat` mask:
AssertionError: 'Wed, 26 Nov 2014 13:19:44 EST' === 'Wed, 26 Nov 2014 13:19:44 GMT--500'
at Context. (/Users/lenny/dev/node-dateformat/test/test_formats.js:64:14)
at Test.Runnable.run (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runnable.js:217:15)
at Runner.runTest (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:373:10)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:451:12
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:298:14)
at /Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:308:7
at next (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:246:23)
at Immediate._onImmediate (/Users/lenny/dev/node-dateformat/node_modules/mocha/lib/runner.js:275:5)
at processImmediate [as _immediateCallback] (timers.js:368:17)
```