Add support for a simple timer to do periodic tasks.

Add support for a simple timer to do periodic tasks.

Base support is on EventLoop, who handles the timer
life and calls handle_timer() when the timer is
expired. dispatcher.Handler has a simple default
handle_timer() implementation that just promote the
"handle_timer message" to all subhandlers, while
PyDaemon just start "spreading the voice" by calling
the root handler handle_timer() method.