The avatar services registry now provides a convenience method to lookup

an avatar service directly by its avatar service ID (as that is all that

is used), as well as a method to get either the requested service or the

default.
In addition, the get_defaults() method has been refactored to reference

a list of avatar service classes on the registry itself to simplify

subclass implementation.
The enabled_services setter now also computes the sets of avatars

services to disable and enable and disables (or enables) each one

individually.

Ran unit tests.