Allow SASL mechanism to be set when creating a ClientXMPP instance.

Instead of using:

    ClientXMPP(jid, password, plugin_config={
        'feature_mechanisms': {'use_mech': 'SOME-MECH'}})

You can use:

    ClientXMPP(jid, password, sasl_mech='SOME-MECH')

If you need to change the mechanism after instantiation, use:

    xmpp['feature_mechanisms'].sasl.mech = 'SCRAM-MD5'

sleekxmpp/clientxmpp.py

@@ -59,7 +59,7 @@ class ClientXMPP(BaseXMPP):
     """
 
     def __init__(self, jid, password, ssl=False, plugin_config={},
-                 plugin_whitelist=[], escape_quotes=True):
+                 plugin_whitelist=[], escape_quotes=True, sasl_mech=None):
         """
         Create a new SleekXMPP client.
 
@@ -114,9 +114,10 @@ class ClientXMPP(BaseXMPP):
 
         # Setup default stream features
         self.register_plugin('feature_starttls')
-        self.register_plugin('feature_mechanisms')
         self.register_plugin('feature_bind')
         self.register_plugin('feature_session')
+        self.register_plugin('feature_mechanisms',
+                pconfig={'use_mech': sasl_mech} if sasl_mech else None)
 
     def connect(self, address=tuple(), reattempt=True, use_tls=True):
         """

sleekxmpp/test/sleektest.py

@@ -293,7 +293,8 @@ class SleekTest(unittest.TestCase):
     def stream_start(self, mode='client', skip=True, header=None,
                            socket='mock', jid='tester@localhost',
                            password='test', server='localhost',
-                           port=5222, plugins=None, plugin_config={}):
+                           port=5222, sasl_mech=None,
+                           plugins=None, plugin_config={}):
         """
         Initialize an XMPP client or component using a dummy XML stream.
 
@@ -317,7 +318,9 @@ class SleekTest(unittest.TestCase):
                         are loaded.
         """
         if mode == 'client':
-            self.xmpp = ClientXMPP(jid, password, plugin_config=plugin_config)
+            self.xmpp = ClientXMPP(jid, password,
+                                   sasl_mech=sasl_mech,
+                                   plugin_config=plugin_config)
         elif mode == 'component':
             self.xmpp = ComponentXMPP(jid, password,
                                       server, port,