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
added tableViewShouldAnimateDraggedCells: data source protocol method, fixed unexpected non-editable cells issue (tableView:editingStyleForRowAtIndexPath: vs tableView:canEditRowAtIndexPath:)
Merge pull request #4 from taberrr/master

fixed issue #3 and some other tweaks