Extensions via entry points

- auto-loading of modules in the 'docker_registry.extensions' entry
  point group
- preliminary documentation
- define and encourage API on top of signals and config

This is an alternative approach to import_modules (#565) and namespace packages (#566).
I'm not able to start the registry unless this file exists (I'm testing locally by running gunicorn directly - not via a Docker image, so I don't know if it would behave differently in that scenario)