EmberScript Generators

Building on the existing code for CoffeeScript generators, this PR along with [this contribution to ember-script-rails](https://github.com/ghempton/ember-script-rails/pull/3) allows for EmberScript files to be generated in place of CoffeeScript. EmberScript is a CoffeeScript dialect that allows for better interoperability with Ember's somewhat unusual architecture. Read more at <http://www.emberscript.com>.

This shouldn't break anything as-is, it just won't work unless ember-script-rails sets the javascript_engine to `:em`.