[Issue #115] Allow subentities to be specified from API response

AFIncrementalStore assumed a one-to-one mapping of API endpoint to entity. However, an API like an activity stream might return several different types of data that should be represented by subentities. With this commit, the API client can map entities to the representations of those entities. The same can be done with relationship representations.

This change should be backwards compatible for those who have not overridden private methods and are using AFRESTClient. For anyone rolling their own client, the following methods are now required:

`representationsByEntityOfEntity:fromResponseObject:`
`representationsByEntityForRelationshipsFromRepresentation:ofEntity:fromResponse:`

And these methods are now optional:

`representationOrArrayOfRepresentationsOfEntity:fromResponseObject:`
`representationsForRelationshipsFromRepresentation:ofEntity:fromResponse:`