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/__init__.py

@@ -12,3 +12,6 @@ from sleekxmpp.stanza.stream_error import StreamError
 from sleekxmpp.stanza.iq import Iq
 from sleekxmpp.stanza.message import Message
 from sleekxmpp.stanza.presence import Presence
+from sleekxmpp.stanza.stream_features import StreamFeatures
+from sleekxmpp.stanza.bind import Bind
+