Anonymous authentication

Implemented ANONYMOUS authentication on the ClientXMPP class.
To use it, you just need to provide a domain (e.g 'anon.example.com')
with an optional resource (e.g 'anon.example.com/resource') as the JID,
with no password. The JID class has been improved to accept
domains as fulljid.

You can test this with echo_client.py
python echo_client.py -j anon.louiz.org/  # anonymous with a resource
                                          # defined by the server
python echo_client.py -j anon.louiz.org/resource  # anonymous with given
                                                  # resource

The "normal" authentication method still works exactly like before.

sleekxmpp/xmlstream/jid.py

@@ -94,6 +94,14 @@ class JID(object):
         elif name in ('server', 'domain', 'host'):
             self.domain = value
         elif name in ('full', 'jid'):
+            if '@' not in value:
+                if '/' in value:
+                    d, r = value.split('/', 1)
+                    object.__setattr__(self, "_resource", r)
+                else:
+                    d = value
+                object.__setattr__(self, "_domain", d)
+            else:
             self.reset(value)
         elif name == 'bare':
             if '@' in value: