API to list work requests

Stefano noted in !409 (merged) that there is no API to list work requests, and so he had to parse manage.py list_work_requests output. We should fix this.

We already have POST /api/1.0/artifact/ to create an artifact, and GET /api/1.0/artifact/<int:artifact_id> to get a single artifact. The obvious thing to do seems to be to add GET /api/1.0/artifact/ to list artifacts, using django-rest-framework's generic list API view. This is an unbounded collection that is likely to grow large over time, so we must have pagination; it would also be reasonable to have filtering, though I don't think that's necessary right away.