Add integration with restlib2 and django-preserialize for defining REST endpoints

This implements #1. One changes is that the resource that backed this URL structure `/<pk>/<op1>/<pk2>/[<op2>/<pk3>/, ...]` was not implemented. Instead the `SetsResource` and `SetResource` supports POST requests that take a series of operations to be applied to the target instance. In the former resource, this will be a new instance while the latter will be an existing one. This provides a high amount of flexibility for creating new sets since operations support logical AND, OR, and XOR as well as subtracting the right set from the left. Further sets can be either a list of object IDs or a primary key of a set instance.

Fix #1