Introduce a custom `keyForHasMany()` for `DS.RESTSerializer`.

This method returns the singularized key appended with `_ids`. It uses the same plurals hash on the serializer to determine singular versions of keys.

Fixes #710
Merge pull request #727 from dgeb/rest-adapter-has-many-ids

Introduce a custom `keyForHasMany()` for `DS.RESTSerializer`.