First pass at re-worked stream features.

Stream features now use stanza objects!

Features are given a ranking that expresses the dependency
relationships (since only one feature is negotiated at a time, the
dependency graph can be replaced by a line).

>>> xmpp.register_feature('my_feature', _my_handler,
>>>                       restart=True,   # Requires stream restart
>>>                       order=600)      # Ranking (out of ~ 10,000,
>>>                                       # lower #'s executed first)

SASL mechanisms may now be added or disabled as needed. Each mechanism
is given a priority value indicating the order in which the client
wishes for mechanisms to be tried. Higher priority numbers are executed
first.

>>> xmpp.register_sasl_mechanism('SASL-MECH', _mech_handler,
>>>                              priority=0)

Disabling a SASL mechanism:

>>> xmpp.remove_sasl_mechanism('ANONYMOUS')

sleekxmpp/stanza/tls.py

@@ -0,0 +1,50 @@
+"""
+    SleekXMPP: The Sleek XMPP Library
+    Copyright (C) 2010  Nathanael C. Fritz
+    This file is part of SleekXMPP.
+
+    See the file LICENSE for copying permission.
+"""
+
+from sleekxmpp.stanza import StreamFeatures
+from sleekxmpp.xmlstream import StanzaBase, ElementBase
+from sleekxmpp.xmlstream import register_stanza_plugin
+
+
+class STARTTLS(ElementBase):
+
+    """
+    """
+
+    name = 'starttls'
+    namespace = 'urn:ietf:params:xml:ns:xmpp-tls'
+    interfaces = set(('required',))
+    plugin_attrib = name
+
+    def get_required(self):
+        """
+        """
+        return True
+
+
+class Proceed(StanzaBase):
+
+    """
+    """
+
+    name = 'proceed'
+    namespace = 'urn:ietf:params:xml:ns:xmpp-tls'
+    interfaces = set()
+
+
+class Failure(StanzaBase):
+
+    """
+    """
+
+    name = 'failure'
+    namespace = 'urn:ietf:params:xml:ns:xmpp-tls'
+    interfaces = set()
+
+
+register_stanza_plugin(StreamFeatures, STARTTLS)