Add support for incoming/outgoing filters.

A filter accepts and returns a stanza, but potentially modified.

To prevent sending/receiving a stanza, a filter may return None.

Incoming:
    self.add_filter('in', in_filter)

Outgoing:
    self.add_filter('out', out_filter)

Filters are applied in the order thay are added. However, you may
add an order parameter, which is the place in the list to insert the
filter:

    self.add_filter('in', in_filter, order=0)

sleekxmpp/xmlstream/stanzabase.py

@@ -1251,7 +1251,7 @@ class StanzaBase(ElementBase):
                          stanza sent immediately. Useful for stream
                          initialization. Defaults to ``False``.
         """
-        self.stream.send_raw(self.__str__(), now=now)
+        self.stream.send(self, now=now)
 
     def __copy__(self):
         """Return a copy of the stanza object that does not share the