Split out handlebars from ember-template-compiler; now we use handlebars...

...-source gem instead of raw vendored handlebars.js file

This is related to this [ember-dev commit](https://github.com/emberjs/ember-dev/commit/8f4637a23a1ede9cc324e656e5deb5fa5b4562b4), and part of a large to combat ruby-js dependency hell that results from different gems packaging their own vendored handlebars.js file. From now on, gems that need handlebars.js (most likely for precompilation purposes) should add `gem 'handlebars-source'` to their Gemfile use `Handlebars::Source.bundled_path` or `.runtime_bundled_path` to get a path to the handlebars.js file they should be using instead of their own packaged version. 

Also removed the no-longer-necessary `lib/ember.js`, which probably hasn't been necessary for a few weeks now.