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

**WIP: do not merge!!!** 
maybe we should keep a single frame for now? the idea of 3 frames before was to already have the busytimes on the cache and speed up the swipe.. I kept it because my plan is to add the same *drag* we have on week/day views to the month view as well and having this logic in place would make it simpler, but maybe we don't even need the 3 frames anymore (since flame is considerably faster than hamachi)