Relationships API

This utilizes the new API extensions that allows services to register their own API bits.

Usage of the API looks like this:

http://localhost:8000/api/v1/relationshipsservice/?ctype=Sample&cid=53cdd9e4d6fa25b059c54ddc&depth=1&types=Sample

The ctype and cid parameters are the CRITs type and ID of the starting object. These are required. The depth and types arguments are optional. If depth is not given it defaults to 3, and if types is not given it defaults to all top-level object types.

As is true of the graph in the UI, if the type of an object does not match the list in the types parameter it is still collected and included in the graph, but is marked as invisible. This is so that the path to other objects related
to it is easy to follow.
Might want to name this something better.