WIP: 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 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