First piece of repository domain implementation

Adds DB API for creating, deleting, and retrieving repository
domains. No support yet for the visibility attribute switching
on retrieval. Will add that functionality after implementing
domain updates and routing into the REST API.

Partially implements Issue #41
Adds ability to update a domain in DB API

Allow a domain to be updated, including checks on transferring
ownership from one user to another.
Merge pull request #44 from jaypipes/domains

Add domain DB API