Resolve many-to-many operations

[joelalejandro]

• GET should bring related resources from either end of the relationship (A->B, B->A)
• POSTing a resource with hould automatically create the record in the intermediate table (AB)
• PUTting a relationship in a resource should automatically create the record in the intermediate table (AB)
• DELETEing a resource should automatically remove the record in the intermediate table (AB)