Date and Datetime transforms

[dustinfarris]

These two transforms allow the user to use date and datetime types in their models.

App.MyModel = DS.Model.extend({
  birthday: DS.attr('date'),
  nextMeeting: DS.attr('datetime')
});

The integration tests are potentially problematic since the JS Date object is time-zone aware—tests may fail when run in timezones other than eastern. I will address this in a future PR.

[dustinfarris]

Travis is failing.. guess I'll have to clear out those timezone issues sooner than I thought.