fixed issue #3 (added some respondsToSelector checks for non-required methods such as tableView:heightForRowAtIndexPath: and numberOfSectionsInTableView:), minimumLegalDistance now takes contentInset.top into account, now passing source indexPath to tableView:didEndDraggingCellToIndexPath:placeHolderView:, added UIGestureRecognizerStateCancelled to gesture recognizer