Fix #628 and #636

[aesedepece]

This parses incoming <presence> stanzas and pass them as PresenceProtocolEntity to upper layers.

Usage example:

# This goes inside your app's YowInterfaceLayer
@ProtocolEntityCallback("presence")
def onPresence(self, entity):
  _type = entity.getType()
  _from = entity.getFrom()
  last = entity.getLast()
  print "Got presence of type %s from %s" % (_type, _from)
  if last is not None:
    print "Last seen: %s" % last