Bug 1084014 - [Calendar] update Month view to use DayObserver r=gaye

**WIP: do not merge!!!** 
the old view was querying for the busytimes during creation, with the new implementation it only adds the listener when it's activated.. so the cache is probably not making a huge difference anymore - still caches the days/markup but the expensive part is querying the busytimes/event data (`timeController.findAssociated`)