More interaction with supplementary views.

[kmisiura]

Added delegate messages about supplementary views being added and removed from document view:

- (void)collectionView:(JNWCollectionView *)collectionView
willDisplaySupplementaryView:(JNWCollectionViewReusableView *)supplementaryView
                ofKind:(NSString *)kind
            forsection:(NSInteger)section;

- (void)collectionView:(JNWCollectionView *)collectionView
didEndDisplayingSupplementaryView:(JNWCollectionViewReusableView *)supplementaryView
                ofKind:(NSString *)kind
            forsection:(NSInteger)section;

Implemented methods to get visible supplementary views:

- (NSArray *)visibleSupplementaryViews;

- (NSArray *)visibleSupplementaryViewsForKind:(NSString *)kind;

[jwilling]

Thank you for putting this together, your contribution is much appreciated. :sparkles:

Just a few pedantic comments. Also, it looks like this might need a merge from head to pull cleanly. Thanks!

[kmisiura]

Sorry for taking this long. Cleaned up code and merged from head.