Added DS.DebugAdapter which extends Ember.DataAdapter

This adds `DS.DebugAdapter` which extends `Ember.DataAdapter` (see https://github.com/emberjs/ember.js/pull/3129).

This will allow inspection of models in the Chrome Ember extension (see https://github.com/tildeio/ember-extension/pull/38).

I kept ED backwards compatible with previous versions of Ember (at least until Ember 1.0).

The build will currently fail until the Ember PR is merged, and the gem version is bumped.