add ember-script support

As titled. Ref: [emberscript](http://emberscript.com/)

I have come across a project where I am heavily developing in [ember.js](http://emberjs.com/), and I adopted emberscript for the library.

I'd like to use emberscript in `jasmine-headless-webkit` too, but it seems that the supported Sprockets engines are hard-coded. I tried adding to it only in userland code, but it ended up being a large hack, cross-referencing and monkey-patching `jasmine-headless-webkit` itself all over the place.

This PR adds emberscript support by hardcoding it into the library, just like how coffeescript is done. Not sure if I have done it The Right Way though, basically I just copied everything that is coffeescript-related, and made an emberscript version for it.