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).
Or very bluntly:

```
def boot():
    for ep in pkg_resources.iter_entry_points('docker_registry.extensions'):
        ep.load()

def list():
   return [ep for ep in pkg_resources.iter_entry_points('docker_registry.extensions')]
```