Add method to register for notifications

Currently this is only implemented client-side, but essentially we
serialize a BUXTON_CONTROL_NOTIFY message for the given key, and
send this off to bt-daemon. bt-daemon (soon) will then use this
to identify clients when key-values are changed, to send a message
indicating that this has happened.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>