Porting to Python 3 left these methods returning unicode, which is
arguably an API break in Python 2:

* Message.get_member
* Message.get_path_decomposed (array of unicode)
* Message.get_sender
* Message.get_destination
* Message.get_interface
* Message.get_error_name
* Server.get_address
* Server.get_id

Instead, make them return whatever the natural str type is (bytes in
Python 2, unicode in Python 3).

2, since there are still some unconditional PyInt calls, which are not valid
in Python 3.  However, it lays the framework for conditionalizing on Python 3
and using only PyLong in that case.  Where it doesn't matter, PyLong is used
unconditionally.

PyString API.  This makes the code compilable in Python 2.x (x >= 6) and
Python 3.

Now that we have the LibDBusConnection temporary objects, we don't have
to do strange sideways inheritance, we can just chain up to the superclass
constructor.

* Move get_unique_name to Connection (it can be useful for connections which
  aren't to a real bus daemon but partially emulate one, like Telepathy's
  Tubes)
* Add set_unique_name to Connection (same reason)
* Convert BusImplementation.__new__ into Connection._new_for_bus
* Have dbus.Bus subclass _dbus_bindings.Connection directly