Phantomjs Headless Runner

Remove jasmine-headless-webkit runner (which is unmaintained) in favor of [PhantomJS](http://phantomjs.org) runner.

### Asset Pipeline
Full integration into asset pipeline and 100% reuse between "in browser" and "headless" experiences.

### Browser Testing
`http://myserver.local/jasmine` is a Rails engine that can be used to run the javascript testsuite locally

### CLI/Headless Testing
`rake spec:javascript` runs the javascript testsuite with a headless webkit instance via [PhantomJS](http://phantomjs.org).  The testsuite can be filtered by using the `SPEC` option. ex: `rake spec:javascript SPEC=my_test_case`