Improve Ember.hasMany

* make it non-embedded by default
* allow to pass options (options['key'] specifies key for the
  association)
* use references instead of ids in HasManyArray, this allows adding
  records without an id to HasManyArray
* properly update relationship if HasManyArray was already created and
  data has been updated
* implement getHasMany function which is called when hasMany association
  is fetched
Implement basic version of belongsTo
Fix FixtureAdapter to properly fetch data in findMany