Fixed: delegate method collectionViewDidChangeSelection: is deprecated

Previously CPCollectionView supported a delegate method collectionViewDidChangeSelection:, which is not defined in Cocoa. Since CPCollectionView is KVO-compliant for selectionIndexes, this delegate method is no longer necessary and is a needless divergence from Cocoa.

The method is now marked as deprecated, and when used will log a deprecation warning.

Closes #715