Support other authentication methods

[acrisci]

The spec for dbus defines several authentication methods but we only use EXTERNAL. And worse, each MessageBus implementation must implement the logic for all auth methods itself.

It would be nice if we could:

1. Create a common class to abstract auth logic from the message bus. This class should extend from a base and can be passed to any MessageBus constructor.
2. Make the base part of the public api so users can extend it to use custom auth code.
3. Implement DBUS_COOKIE_SHA1 and ANONYMOUS auth method classes included in the library.