sockets: implement automatic loading of namespaces into view

[adamantivm]

On the server side, gevent-socketio implements that concept by providing a class BaseNamespace, which we extend, and later setup as the server endpoint. Having our code organized in Django apps, however, means that the namespace class definition is away from the code that set the endpoint name.

We need a way to define BaseNamespace descendants and pass them to socketio_manage without having to explicitly import them.