generic event updates

renamed fd-add to watch-fd (fd-add remains for backwards compatbility)

unified timers and fds into "event.lisp".

added event class, wraps libevent c event object. on top of that added
functions free-event, remove-event, add-event.

new event object is returned from both delay and watch-fd now, meaning
you can pause/free events by hand.

renamed timer.lisp -> event.lisp in tests, although have not added FD
tests YET.