* doc/dbus-sasl-profile.txt: docs on the authentication protocol,
	it is a simple protocol that just maps directly to SASL.

	* dbus/dbus-auth.h, dbus/dbus-auth.c: authentication protocol
	initial implementation, not actually used yet.

	* dbus/dbus-string.c (_dbus_string_find): new function
	(_dbus_string_equal): new function
	(_dbus_string_base64_encode): new function
	(_dbus_string_base64_decode): new function