Implement django-rest-framework.

This is a first cut using d-r-f.

Couple of things...

If I split the API into jenkins/api.py and projects/api.py for example, when I incorporate them into the urls as separate urls routed at the same location, the "api" render doesn't include the second router, so whatever gets listed second doesn't appear.

ManyToMany through isn't great, I'm wondering about whether or not to change this to a normal relationship rather than using the ManyToMany through, as we care about the join-table a lot (see the code).