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
Save relationship names
Serialize relationships in toJSON
Always create a reference for the record
Ensure that embedded hasMany relationship can get new records
Remove console.log
Fix failing reload test
Merge branch 'relationships-improvements'