add method and improve scrolling method to consider if header is pinned.

Suggestions for refactoring out the duplicate code for checking pinsHeaderToTop? It bugs me but so does adding something like pinnedIndexPathForRowAtDate:.  Could also do indexPathForRowAtDate:IgnorePinning: where the second argument is a BOOL on whether to use pinning or not. Or maybe is whether to return the header row for the month instead of the exact row. That doesn't seem like a great solution either.