Enable compatibility with the default Ember {{input}} helper.

* Move the original `{{input}}` helper to `{{ember-input}}` before
  replacing it.
* If EasyForm's `{{input}}` is used without specifying a propertyId
  use Ember's original `{{input}}` helper (now stored in
  `{{ember-input}}`).
Merge pull request #84 from rjackson/allow_input_without_propertyId

Allow usage of Ember's {{input}} helper.