Remove Ember.String.classify since it is provided in Ember.String core

I think `Ember.String.classify` should be removed since it is provided by `Ember.String` itself.

I removed all tests as well. I even removed the test
`equal(Ember.String.classify('words'),'Word');` which I think was
actually an error in the test suite: it should be
`equal(Ember.String.classify('words'),'Words');`. See http://jsbin.com/iwodap/1/edit for more details.