* doc/dbus-specification.xml: document org.freedesktop.DBus.GetId()

	* bus/driver.c (bus_driver_handle_get_id): implement org.freedesktop.DBus.GetId()

	* bus/bus.c (bus_context_new): generate a unique ID for each bus context

	* dbus/dbus-connection.c (dbus_connection_get_server_id): new function

	* dbus/dbus-bus.c (dbus_bus_get_id): new function

	* dbus/dbus-server.c (dbus_server_get_id): new function