Improved docs

- Fixed an outdated URL
- Rewrote "Separate Celery" section
- Changed literal blocks to bash code-blocks
- Changed wording when referring to the MediaGoblin WSGI application
No more of this find_packages nonsense.

Explicitly declare mediagoblin as the only package.  find_packages has
caused messes in the past :)

This commit sponsored by Pedro Ângelo.  Thanks!