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 can add support for embedded polymorphic associations
by overriding`addHasMany` of the serializer.

Fix typos

Support belongsTo associations defined with a type as a string

`polymorphic` can be used with `inverse`

Failing test - can not convert "comment" to `App.Comment`

wip