CollectionViewController: more ergonomic initializer w/ default model/layout/binders

[wkiefer]

From @danielrhammond 's comment in https://github.com/dropbox/pilot/pull/109 :

random idea, but since we do a lot of things like this in our app it might be nice to provide an optional protocol CVCs can conform to with some static funcs which if conformed to provide a convenience init(context:) initializer

static func defaultModel() -> ModelCollection static func defaultLayout() -> NSCollectionViewLayout static func viewBinder() -> ViewBindingProvider