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 really splitting hairs, so don't think there's anything wrong with what you did. It just seems that what you're trying to do is take what ``iter_entry_points()`` returns and make it a list. If that's all you want, then just casting it as a list is the simple and direct thing to do.

However, if you want to do item transformation, then a list comprehension would be useful. You could, for example, return ``[ep.module_name for ep in pkg_resources.iter_entry_points('docker_registry.extensions')]`` to return a list of the module names instead of a list of ``EntryPoint`` objects.

But like I said, nothing about what you did is incorrect. I'm just butting in with a minor drive-by unsolicited suggestion. ;)