* [ASScrollNode] Support for automaticallyManagesContentSize, adopting the ASLayoutSpec's size as the scrollable contentSize.
This feature has been desired for a long time, and has turned out to be phenomenally useful and easy to use.
It works well either for a blank ASScrollNode with .layoutSpecBlock set on it, or a subclass of ASScrollNode with a more
traditional layoutSpecThatFits: implementation. With this approach there is no need to capture the layout size, use
an Absolute layout spec as a wrapper, or set contentSize anywhere in the code and it will update as the layout changes!
There is no automatic management of contentInset, but it would make sense to add this with keyboard listeners in the future.
* [ASScrollNode] Add locking to new properties, adjust how calculateLayout override is done.
Closes#99. This is a quick sketch -- room for improvement includes
considering a less-questionable class name, potentially exposing
additional UIScrollView functionality directly on the node, and using it
in a sample project.
