Helper function that reliably verifies if a given function is a view "class" (or view constructor)

If we want to proceed with the suggestions given in issue #1846, this helper function is the first step. But I think it is quite useful by itself.

It would allow us to have dynamic view classes (the view class would be obtained at run-time). The first place to use this would be in CollectionView, but I see it being useful in other parts of Marionette. 
Maybe `fn` instead of `f`.