reviewed by: <delete if not using a buddy>

	* dbus/dbus-internals.c: (_dbus_type_to_string):
	New function that returns a string describing a type.

	* dbus/dbus-internals.h:
	* dbus/dbus-message.c: (dbus_message_append_fields),
	(dbus_message_append_fields_valist), (dbus_message_get_fields),
	(dbus_message_get_fields_valist), (_dbus_message_test):
	* dbus/dbus-message.h:
	Add new convenience functions for appending and getting message fields.
	Also add a test for those.