Add Ember.Date.parse to fix cross browser bugs parsing ISO-8601 dates
Added support for polymorphic associations.

App.User = DS.Model.extend({
 messages: DS.hasMany(App.Message, {polymorphic: true})
});

App.Message = DS.Model.extend({
  created_at: DS.attr('date'),
  user: DS.belongsTo(App.User)
});

App.Post = App.Message.extend({
  title: DS.attr('string')
});

App.Comment = App.Message.extend({
  body: DS.attr('string'),
  message: DS.belongsTo(App.Message, {polymorphic: true})
});

You need to configure the serializer to map to the correct type:

DS.RESTAdapter.configure('App.Post' {
  alias: 'post'
});
DS.RESTAdapter.configure('App.Comment' {
  alias: 'comment'
});

Initializing an alias will automatically allow you to sideload this
type. The support for sideloadAs is only for backward compatibility.

Two hooks exist for the serialization and materialization of an embedded
polymorphic association:
* `addType` that adds the record's type to the serialized data
* `extractEmbeddedType` that retrieves the record's type from the
payload.

Both hooks rely in the JSONSerializer on `keyForEmbeddedType`, which returns the string used to
represent the record's type in the payload. The default value is
`'type'`
Merge pull request #750 from Cyril-sf/polymorphic_associations

Added support for polymorphic associations.
Merge pull request #723 from klaaspieter/date-fix

Add date shim to fix cross browser issues with parsing ISO-8601 dates