core (xmpp): client.roster now implements __contains__

Now a jid can be tested directly with "entity_jid in client.roster".

sat/core/xmpp.py

@@ -1303,6 +1303,9 @@ class SatRosterProtocol(xmppim.RosterClientProtocol):
         self._jids = {}  # map from jids to RosterItem: key=jid value=RosterItem
         self._groups = {}  # map from groups to jids: key=group value=set of jids
 
+    def __contains__(self, entity_jid):
+        return self.isJidInRoster(entity_jid)
+
     @property
     def versioning(self):
         """True if server support roster versioning"""