It was difficult in the past to override serialization of objects in

payloads, but there are times when you really want this.
Now, instead of directly looking up a serializer from the global

registry for a given object, we go through a function on the resource

itself. This defaults to looking up in the registry, but the resource

can override this in order to override any object it chooses that would

be represented in its payload.
Likewise, sometimes a resource needs to override the parent URL

computation for an object. It could already do this before, but it

didn't have access to all the variables that made up the URL. We now

pass **kwargs, like we do with most other similar functions.

Djblets and RB unit tests pass.
Made use of both the new additions in a new set of APIs for RBCommons.