Setup globals via karma.conf.js

It would be helpful to allow karma to setup globals in karma.conf.js via a `globals` object.
This would be, for instance, helpful with tests that run agains an API (like contract tests).

If the configuration is kept in a json file (common scenario), it currently requires a trickery like this:

``` JS
var fs = require('fs');
var MY_GLOBAL_API = 'some global value';
fs.writeFileSync('globals.js'), 'apiEndpoint = "' + MY_GLOBAL_API + '";', 'utf8');

module.exports = function(config) {

var configuration = {
files: [
'globals.js'
.....
```

It would have been a bit easier with:

``` JS
var MY_GLOBAL_API = 'some global value';
module.exports = function(config) {

var configuration = {
globals.apiEndpoint = MY_GLOBAL_API;
.....
```

Sort of similar to [how](https://github.com/angular/protractor/blob/master/docs/referenceConf.js#L255) protractor does it. I'd be happy to work on a PR.