Add key fingerprint to presence broadcasts (#61)

This is a "workaround" for key rollover management on clients.
Clients will receive user key fingerprint after every presence
subscription along with his/her presence, so clients can check
if it's changed. If it is, clients will request his/her vCard
with the new key.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>