Update Readme unit test section
add fixture serializer
use adapter interface in fixture adapter
Merge pull request #716 from tchak/fixture-adapter-cleanup

Fixture adapter cleanup
Implement extractHasMany and extractBelongsTo
Merge pull request #718 from tchak/fixture-adapter-cleanup

Implement extractHasMany and extractBelongsTo on FixtureAdapter
Merge pull request #713 from bradfol/readme-tests

Update Readme Unit Test section
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