Feature request: moment transform

[jamesarosen]

It'd be lovely to have an ember-data transform so I can make all model date fields moments:

export default DS.Model.extend({
  createdAt: DS.attr('moment'),
  name: DS.attr('string'),
  updatedAt: DS.attr('moment'),
})

[jasonmit]

@jamesarosen this was asked for in the past. @stefanpenner was against it because of the perf impact.

Context: #157 (comment)