Fix#14 Prevent from a crash when no footer or no header.

This methods should return nil for no footer or no header:
```
- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath;
```
And this method, declared in UICollectionViewDatasource (implemented in the Demo project) :
```
- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionReusableView *view = nil;

    view = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"HeaderView" forIndexPath:indexPath];
    
     if ([kind isEqualToString:UICollectionElementKindSectionFooter]) {
        view.frame = CGRectZero;
    }
    return view;
}
```
Note: The crash is not in this library, but in _createPreparedSupplementaryViewForElementOfKind: which is a private API of UICollectionView.