Code review comment for lp:~zeitgeist/zeitgeist/zeitgeist.client.extensions_interface

Add a get_extension method to client.ZeitgeistDBusInterface to easily access interfaces
of extensions.

- The generic stuff from ZeitgeistDBusInterface is refactored into a new
  class, client._DBusInterface, so that it can also be used by the extensions.
- Fix client._DBusInterface.connect() so that it can take any number of argX=
  arguments.
- Remove get_session_bus. dbus.SessionBus() is a singleton.
- blacklist-test.py: add a test-case using the new get_extension() method