Initial REST API endpoints for repos and branches

This sets up ReST an initial set of endpoints for repos and branches. Little work, but it is already a full-fledged API thanks to the magic of DRF. This will be the starting point to doing any Angular front-end.

You can browse the API (and create objects as well) at http://localhost:8000/repos/repo/

Not yet included in this PR are user authentication and permissions (it will be the purpose of https://txels.tpondemand.com/entity/275 and later stories, and this PR is still part of https://txels.tpondemand.com/entity/282)