Support sdbus clients (#54)

This is built on top of
#26. I've kept the
original commit by @refi64 and added one to address the [issue raised
during the
review](#26 (comment)).

Whether my commit actually succeeds in addressing that problem is
something I'm not sure of, since I couldn't actually reproduce the
original issue. But from my first tests it appears that sdbus clients
can connect and send methods calls.

Possible fix for #21