Refactor ItemView references to ChildView

I took a stab at refactoring `CollectionView` and `CompositeView` this weekend to consistently refer to child views. This is a pretty high touch change, so I'd appreciate as many eyes as possible. Once everything looks good, I'd be happy to go into the docs. 

With something like this, I think it'd also be good to have a corresponding upgrade doc that discusses the changes and what you'll need to do to change it over.


[fixes #447]