AUI-1546 Fixing the 'Show x more' link logic in month view

As far as I understood the `_getRenderableEvent` method, an event which starts the first day of a week isn't rendered in that week if the first day is already full. I had a look at Google Calendar, and they seem to do the same.

With this change, an `evtRenderedStack` entry contains all the events that are rendered from that specific day, plus a number of null values for the events the span in that day (we don't need to store the actual events, just the number of them). Then, we show the "Show x more" link if we're rendering the last available row and the number of events we rendered in that day is less than the number of events we should show.