Bring Ember.literal in as a dependancy

There's at least one repo that could benefit from `Ember.literal`, but doesn't want to introduce a dependency on ember-cpm to get it.

https://github.com/stefanpenner/ember-moment/pull/10#discussion_r19051029

I broke `literal` out into its own repo ([TrueNorth/ember-literal](http://github.com/TrueNorth/ember-literal)) and would like to bring it in to ember-cpm as a dependency