Add default to numberOfSectionsInCollectionView: method

This branch will use the Apple specified default for `numberOfSectionsInCollectionView: ` when not explicitly defined on the datasource. See: https://developer.apple.com/library/ios/documentation/uikit/reference/UICollectionViewDataSource_protocol/Reference/Reference.html#//apple_ref/occ/intfm/UICollectionViewDataSource/numberOfSectionsInCollectionView:

* [x] Check for existence of `numberOfSectionsInCollectionView:` on collection view datasource
* [x] Use default specified by Apple docs if not found
* [x] Ready for review