Merge pull request #87 from drogus/get-attr

Add getAttr hook
Second argument in hasMany should be an options hash

That way it will be easier to extend functionality, like for example add
ability to decide if relationship is embedded or not.
Add non-embedded hasMany relationship (read-only)
Create a reference for each model

Reference keeps basic information about the model: clientId, primaryKey,
type of the record and a reference to the record itself. It's useful
when we can't use ids to keep reference to the record (for example for
new records).
Change has_many/object_load_test to use Ember.hasMany

hasMany method's functionality will be changed to keep references at a
content, so it will be wiser to use Ember.hasMany instead of custom
computed property
Keep references instead of ids in HasManyArray's content