(bus_driver_generate_introspect_string): New method for populating
  a DBusString with the introspect data
  (bus_driver_handle_introspect): Move introspect generation code to
  bus_driver_generate_introspect_string

* bus/main.c:
  (introspect): New function which prints out the intropect data and
  exits
  (main): Add a --introspect switch