Allow getAll to handle objects wrapping elements

Summary:
 * We can now not only accept a response with a raw json array,
     but also with such array wrapped in a JSON object.
    * The key under which such array is located, defaults to "elements", but
        can be set through a new setter.
    * Setting such key to null, will preserve the old behavior

Linting

Test Plan: Load a resource and getAll, both wrapped and unwrapped

Reviewers: mpurita, jmsotuyo

Reviewed By: jmsotuyo

Differential Revision: http://ph.monits.com/D6625