extract Ember.isEmpty into ember-metal

Ember computed.empty should always have identical semantics
Add Ember.computed.none… none + empty have different secants.

Also extract Ember.isNone into ember-metal
Ember.isArray implementation lives in ember-metal, its tests should also

Add test of Ember.isArray against an arrayProxy.
Merge pull request #2228 from stefanpenner/improve_many_small_things

Improve many small things