XMPP Backend

[wallabra]

The triarc bot's XMPP backend will, as the name implies, enable triarc bots to also work in the XMPP environment.

This will require a new Backend implementation, complete with its own implementations of MessageProxy, UserProxy and ChannelProxy (MUCs?), and a Messageable implementation. Like with IRC, we may be fine with just one Messageable implementation for user and channel addresses alike, as they are both interchangeable; here, it's because they're both addresses of some sort.

• Implement basic backend types (including Messageable and proxy types)
• Basic XMPP connection ability
• Start and stop handlers; finish abstract Backend methods
• Expose some functonality with MessageProxy, UserProxy, ChannelProxy, Messageable
• (after config) Test using the pingbot example