Add support for writing AppRTC javascript unit tests.

This uses js-test-driver rather than JsUnit which is Google standard. Unfortunately, there don't seem to be any grunt plugins or support for JsUnit. At least js-test-driver is very similar.

The solution is based on grunt-jstestdriver which uses PhantomJS to bring up a WebKit-backed browser. This means unit tests probably will not be able to actually use WebRTC. They will be able to test the business logic of AppRTC though, such as the example sdp util test I added below. It might be possible to test against Chrome if we somehow got that installed on the Travis boxes, but I wouldn't count on it. Arguably, any test that actually invokes WebRTC js is probably an integration test anyway.
Add a comment as to why we have to do this.