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

**WIP: do not merge!!!** 
`_createFrame` is somewhat misleading since it's possible that it's coming from the cache (and not being created). Do you think `_getFrame` or something similar could be better? I read `changeDate` and my first question was whether we might have already created the frames for previous and next month. You actually did handle that case since this uses a cache, but the name tripped me up.