Flesh out repository deletion and add basic search indexing

This addresses #45 by fixing the `delete_repository` URL [1][], filling in tag-deleted signals, and triggering a new repository-deleted signal.  It also adds repository-created and repository-updated signals.  With the signaling in place, it takes a first pass at a `GET /v1/search` implementation (#63).  If this seems like a productive line, I can replace the current stub Index class with something based on ~~Redis (which would be trivial) or~~ SQLAlchemy (which would be a bit more work, but would allow efficient search, insertion, and deletion for large registries).  No new tests yet, or ~~any way to disable the indexer~~, but it's working for me on my local registry.

[1]: https://github.com/dotcloud/docker-registry/issues/45#issuecomment-34828540