More interaction with supplementary views.

<h5>Added delegate messages about supplementary views being added and removed from document view:</h5>
```objectivec
- (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;
```

<h5>Implemented methods to get visible supplementary views:</h5>
```objectivec
- (NSArray *)visibleSupplementaryViews;

- (NSArray *)visibleSupplementaryViewsForKind:(NSString *)kind;
```