fix dbus-single-signature?

When passed anything else than a string it blew up, which is something a
type predicate should never do.  This fixes the issue.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
added dbus-get-properties

An utility function that takes a proxy object implementing
dbus-properties<%> and dbus-introspectable<%> and returns all of it's
interfaces' properties along with their respective values.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>