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).
Loose contract is fine for now. We'll see after 0.9 if it's worth it making things more strict.

Can we have a `list` method in there that would return all available extensions?