Do not require models to be an Ember.Object.

* Changes tests to use POJO's instead of an Ember.Object.
* Changed Ember.EasyForm.Button and Ember.EasyForm.Submit to use
  `isValid` on the model instead of `isInvalid`. This makes it easier
  to implement custom validations (without `ember-validations`) since
  only one value is needed for checking validity.

Resolves #87.