Fix issue preventing display of supplementary views when registered from a nib

When a supplementary view is registered via -[JNWCollectionView registerNib:forSupplementaryViewOfKind:withReuseIdentifier:], the collection view would never display it.

The cause was that supplementaryViewNibMap was never including when searching for supplementary view identifiers.
Merge pull request #116 from nerdyc/supplementary-nibs-fix

Fix issue preventing display of supplementary views when registered from...