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).
Something like this?

```
extensions = []

def boot():
    for ep in pkg_resources.iter_entry_points('docker_registry.extensions'):
        extensions.append(ep)
        ep.load()

def list():
    return extensions
```