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).
No, it's under openshift. You do at least need an empty `__init__.py` (but not one that specifies the namespace package setup). I'll update.