Upgrade gunicorn to 19.1

* gunicorn added timezone logs in v19.0 [1]
* Altered docker-registry logging to include timezone
* Altered documentation to more closely reflect implementation (-w and --max-requests flags as per use in run.py)
* Removed --debug flag, deprecated in v19.0 [1]
* Removed unecessary documentation from wsgi.py (we already suggest how to run Gunicorn in the README)
* Mention GUNICORN_OPTS in run.py's docstring, which should have
  happened in 2008330 (docker_registry.run: Restore GUNICORN_OPTS
  functionality, 2014-07-18).

[1]: http://docs.gunicorn.org/en/19.0/news.html#core