Add support for OAuth scopes to OAuth2 authenticator

This PR adds support for OAuth scopes as described in [Resource Owner Password Credentials Grant](http://tools.ietf.org/html/rfc6749#section-4.3.2).

To use scopes one will need to implement the `authenticate ` action on their login controller (instead of using ember-simple-auth's login controller mixin):

```js
import AuthenticationControllerMixin from 'simple-auth/mixins/authentication-controller-mixin';

export default Ember.Controller.extend(AuthenticationControllerMixin, {
  authenticator: 'simple-auth-authenticator:oauth2-password-grant'

  actions: {
    authenticate: function() {
      var data = this.getProperties('identification', 'password', 'scope'); // note the `scope` property
      this.set('password', null);
      return this._super(data);
    }
  }
});
```

closes #355 
The argument should probably not be named `credentials` anymore then.