Add `pin.readableMargins` and `pin.layoutmargins` properties

Add properties:

• pin.readableMargins: UIEdgeInset:
  PinLayout's UIView.pin.readableMargins property expose UIKit UIView.readableContentGuide as an UIEdgeInsets. This is really useful since UIKit only expose the readableContent area to Auto Layout using UILayoutGuide.

• pin.layoutmargins: UIEdgeInset
  PinLayout's UIView.pin.layoutMargins property expose directly the value of UIKit UIView.layoutMargins. The property exists only to be consistent with the other areas: pin.safeArea, pin.readableMargins and pin.layoutmargins. So its usage is not necessary.

Add examples using these properties:

• Added by Luc Dion in Pull Request #170