Many improvements in Watch object: try to reuse as much as possible

Many improvements in Watch object: try to reuse as much as possible
objects.

This introduce a new API for Watch:
 $watch = new Watch($user [, $date]);
 if ($watch->count() > 0) {
   $events = $watch->events();
   ...
 }

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>