Add support for roster versioning.

This was XEP-0237, but is now part of RFC 6121.

Roster backends should now expose two additional methods:

version(jid):
    Return the version of the given JID's roster.
set_version(jid, version):
    Update the version of the given JID's roster.

A new state field will be passed to the backend if an item
has been marked for removal. This is 'removed' which will
be set to True.

sleekxmpp/features/feature_rosterver/stanza.py

@@ -0,0 +1,17 @@
+"""
+    SleekXMPP: The Sleek XMPP Library
+    Copyright (C) 2012  Nathanael C. Fritz
+    This file is part of SleekXMPP.
+
+    See the file LICENSE for copying permission.
+"""
+
+from sleekxmpp.xmlstream import ElementBase
+
+
+class RosterVer(ElementBase):
+
+    name = 'ver'
+    namespace = 'urn:xmpp:features:rosterver'
+    interfaces = set()
+    plugin_attrib = 'rosterver'