Support Ember.isEmpty or Ember.empty

Ember 0.9.8.1 defines `Ember.empty`. Ember 1 deprecates `Ember.empty` in favor
of `Ember.isEmpty`. Though this branch is targeted at Ember 0.9, it can also
be used with Ember 0.9.9-zendesk, which does the same deprecation. Now
`EmberCPM.Macros.empty` and `notEmpty` prefer `isEmpty` and fall back on `empty`.