XMPP certificate support

Currently the XMPP code uses username/ password auth; needs to support certificate login.  This appears to already be supported in Sleek:

https://github.com/fritzy/SleekXMPP/blob/master/sleekxmpp/xmlstream/cert.py
https://github.com/fritzy/SleekXMPP/blob/master/sleekxmpp/plugins/xep_0257/client_cert_management.py
