docker_registry.app: Add a /_versions endpoint

For debugging it's good to know what package versions folks are
running.  This endpoint allows folks using floating dependencies (or
just different versions of docker-registry) to figure out the exact
version of all of their Python dependencies.

The only basic dependency that *doesn't* define a __version__ is
itsdangerous (as of v0.24), which doesn't provide a
programmatically-accessible version number.  I haven't checked all the
peripheral dependencies outside of what gets pulled in by a stock:

  $ pip install depends/docker-registry-core/
  $ pip install .

To avoid accidentally leaking implementation details, the /_versions
endpoint is disabled in the common config_sample.yml block, and only
enabled by default in the dev block.  In both cases, you can use the
DEBUG_VERSIONS environment variable to enable/disable the endpoint for
your particular instance.
Merge pull request #473 from wking/versions-endpoint

docker_registry.app: Add a /_versions endpoint