PSTCollectionElementCategory and layout subclassing

I implemented a layout subclass that required `- (PSTCollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath;` so I moved it public because it is public in `UICollectionViewLayoutAttributes`.

http://developer.apple.com/library/ios/#documentation/uikit/reference/UICollectionViewLayoutAttributes_class/Reference/Reference.html

These changes also fix #243