Automatic Sizing

Add Automatic Sizing feature

By calling autoSizeThatFits with the given available size and a layout closure, any layouting performed by PinLayout in that closure will be computed without affecting any subview's frame in the view hierarchy. On the other hand, any non PinLayout related code will also be executed. For that reason, it is really important to separate your layout code in it's own function to avoid any side effect during sizing, like setting the scroll view's content size in the above exemple or perhaps assigning itemSize in a collection view layout. That kind of code that depends on the layout should only be executed when layoutSubviews() is called as part of a normal layout pass.

The resulting size also takes into account the margins applied on subviews, even on the bottom and trailing sides. Automatic sizing makes it really easy to write your layout logic once and add proper sizing behavior with virtually no additional effort.

See https://github.com/layoutBox/PinLayout#automatic_sizing for more documentation.
* Added by Antoine Lamy in Pull Request #216