Add support for RubyBox::Event#source?

This is probably not worthy of a 1.1.1 release but next time you release it would be awesome if you could add this in. Sometimes an event won't have a source associated with it and therefore gives you a "stack level too deep" error when trying to access it. I added this so you can `ev.source if ev.source?` when accessing it. Maybe there is an easier way to do this but it wasn't evident to me in the item.rb code.

Cheers,

Dan