Allow non-auto-registered complex type

The default behavior of Base is to auto-register subclasses on their
__registry__, which is itself auto-initialized with the default registry.
This is a problem when we want to register the classes on a registry that is
instanciated after the class definition (which is the case in cubicweb-wsme).

With this patch this behavior can be inhibited by setting a 'None' or 'False'
__registry__ to a class.

Change-Id: Ia4bc3d295911b9dbaa8bd855fc0e61634e778a1d
Merge "Allow non-auto-registered complex type"