use EventAdapter to execute ftpserver's callbacks

By using EventAdapter we make sure that all callbacks are executed
in the queue of the worker which bound those callbacks.
one `var` too much can make a big difference

I'm also using JavaEventEmitter which nicely wraps the formerly
used EventAdapter.
document events
Merge branch 'eventadapter' of https://github.com/oberhamsi/ringo-ftpserver into oberhamsi-eventadapter

Conflicts:
	lib/events.js