Add UserKeyResource to API

Implements:

 GET /users/{user}/keys/{fingerprint}
 DELETE /users/{user}/keys/{fingerprint}

Adds plumbing into the WSGI API for getting and
removing user public keys.

Also includes changes to fix errors that were introduced
with some backwards incompatible changes with Falcon:

 falcon.HTTPUnauthorized no longer takes schema as a positional argument
 falcon.api.API.set_default_route is deprecated, so added
 VersionsResource directly as a add_route().