Provide determinate z-order for elements (airbnb#32)

* Explicitly specify layout attributes z-index

* Assign descending z-indices based on number of items

* Update example project with a background in default data

* Make small code style corrections

Evil159/footer support rebased (airbnb#31)

* Footer support.

* Update example project with footer support.

* Update readme.

* Updates for rebase

* Improvements

Enable RTL language support (airbnb#30)

* Enable RTL language support

* Fix build

Bk/fix header sizing assertion (airbnb#29)

* Add header sizing assertion fix

Fix method name

* Fix supplementary view copy-on-write stale data issue

* Fix grammar in previous comment

* Lazily create previous layout attributes as-needed

Remove commented out debug code

* Update version to 1.2.3

* Simplify preferred height update context naming

Introduce a workaround for radar 46865293 (airbnb#28)

Expose MagazineLayout.Default enum as public (airbnb#10) (airbnb#17)

Take safe area insets into account (airbnb#14)

* Take safe area insets into account

* Update comments for safe area insets hint

* Update podspec version to 1.2.0

Bk/fix jumpy scrolling after rotation airbnb#9 (airbnb#11)

* Fix jumpy scrolling after rotation when using self-sizing

* Bump Pod version

* Minor style updates

* Clear out new properties

Initial release

Refactor preferred attributes updating workaround