Add option for sticky month labels in calendar with week scope

[benrudhart]

If calendar is used in a week scope:
The month labels "change" every time you change the week. This can be confusing cause month labels most often will change to the same month (e.g. from January 2016 to January 2016).
When using this option the month label only changes when the month of the date in the current week view changes.

[benrudhart]

Why did you close this PR? Think this could be really helpful bc. it removes the redundant changes of the month labels in .Week scope when the month didn't really change...

[WenchaoD]

When I test it in "FSCalendarScopeExample", the month label stay in the middle of the transition (but the scrolling animation has already stopped), it looks wierd I think.

[benrudhart]

That should only be the case when the week contains parts of two months (here January and February). There should only be one label in case the week contains only days that belong to one month (for instance Februar, if you keep on scrolling to the right to end up in the week starting from February 6th.)
This way the month labels show the same as the days of the week show: parts of two months.

[notDanish]

date = [_calendar dateByAddingMonths:indexPath.item-1 toDate:firstPage];