Remove UIImagePickerControllerDelegate
Remove UINavigationControllerDelegate
Change detecting player paused code.
Move some code into buildSessionForMediaURL in ViewController
Add removeObserver codes to ViewController
Change initial view controller to RootViewController which is subclass of UICollectionViewController
Change initial rate of exporter
Merge branch 'feature/collectionView' into develop
Merge pull request #1 from norio-nomura/master

Add collectionView based UI. Thanks a lot!