Add very basic Stream Management (XEP-0198) support

This implementation only counts incomming stanzas and
sends ACKs on request.
While basic this already allows servers to write
messages to offline storage that were sent to,
but never received by a client.

src/XMPPConnection.m

@@ -361,6 +361,11 @@
 	return supportsRosterVersioning;
 }
 
+- (BOOL)supportsStreamManagement
+{
+	return supportsStreamManagement;
+}
+
 - (BOOL)checkCertificateAndGetReason: (OFString**)reason
 {
 	X509Certificate *cert;
@@ -861,6 +866,10 @@
 			  namespace: XMPP_NS_ROSTERVER] != nil)
 		supportsRosterVersioning = YES;
 
+	if ([element elementForName: @"sm"
+			  namespace: XMPP_NS_SM] != nil)
+		supportsStreamManagement = YES;
+
 	if (mechs != nil) {
 		OFEnumerator *enumerator;
 		OFXMLElement *mech;