Custom XML support

[ForNeVeR]

There should be a support for custom XML sending and responding in the XMPP module. Implementor should think about supporting the SendXML command in the JabberProtocol (rename it to XMPPProtocol maybe?) and providing an XEP-based API in the Handlers (currently there's MucHandler and PrivateHandler but there conceptually may be more of them). Maybe factor out the Handler interface for easier protocol implementing?

[rexim]

I couldn't think of a natural implementation of the ping command because of lack of this feature. Looking forward to this.

[ForNeVeR]

The whole Babbler migration idea finally landed into #410. So this task has a dependency on #410.