NGNinjaTableViewCell: remove private properties from header file, indexPath is no longer retained by the cell.
Cleanup: add new line at the end of file, remove unnecessary import.
NGNinjaTableView+SectionManagement: fix bug that violated encapsulation
NGNinjaTableView+SectionManagement: add sectionForHeaderView: method.
Add NGNinjaTableViewHeaderFooterViewAppearing protocol
NGNinjaTableView+SectionManagement: foldedSectionsIndexSet and hiddenSectionsIndexSet returns NSIndexSet, not NSMutableIndexSet. Fix crash bug.
Fixed few typos
Formatting and License