Merge pull request #366 from ckung/master

Fix nonembedded belongsTo does not have container.
Allow HEAD (undefined response body) on RESTAdapter
Merge pull request #372 from ahacking/rest_adapter_head_response

Support HEAD response (undefined response body) on RESTAdapter
Relationships should not return promises when using the store
Model.fetchAll returns existing promise if there is one outstanding.

Fixes issue #386
Merge pull request #387 from jnovatnack/reuse-fetch-all-promise

Model.fetchAll returns existing promise if there is one outstanding.