Lazily load Net::XMPP in Jabber module

[davidben@mit.edu]

Filing this so we don't forget. We've talked about not loading modules automatically to save memory (Net::XMPP is a pig and slows down startup), but it's probably easier (less mucking about with, say, .owl/startup containing a :jabberlogin without the module explicitly loaded) if the laziness were done within modules. BarnOwl::Module::Jabber should register commands but not load the rest of the module until someone runs one of the commands (or specifically :jabberlogin).

[jgross@mit.edu]

davidben has a branch for this at ​https://github.com/barnowl/barnowl/pull/91