Adds REST API for operations on repository

Adds the following REST API resources for managing repositories:

GET|POST /repos
GET|PUT|DELETE /repos/{repo_id}
GET|POST /domains/{domain_id}/repos
GET /{domain}/{repo_name}

The last REST API above is to enable a more GitHub-like API
retrieval of domains and repositories, including repositories
under a "personal" domain.

Completes Issue #48