Stacktrace improvements

There is a lot of nodeunit and async.js noise in the stacktraces... If we can _reliably_ improve the stacktraces, it would make debugging failing tests a little nicer.

I'm a bit worried about messing with something as fundamental as a stacktrace, but it might be worth experimenting with, and could be included as an option to the command line tool at first.

See: http://code.google.com/p/v8/wiki/JavaScriptStackTraceApi