consolidates pushSinglePayload and pushArrayPayload into pushP...

...ayload. This also allows it to be used through the store's pushPayload.

This PR is a rebased on master version of this original PR: https://github.com/toranb/ember-data-django-rest-adapter/pull/53

This will fix issue #51.

This requires a pull request on ember-data to be pulled: https://github.com/emberjs/data/pull/1474