Minor improvements and fixes to Cartography

[corujautx]

This PR includes improvements and fixes to Cartography 3.0

• Discontinuation of the support for macOS < 10.10: The reason I did this is because Cartography 3.0 makes use of the isActive property on constraints and no longer has views to attach to a context like in previous versions, as those contained traversal of view hierarchy to identify where to attach a constraint as previous APIs did. It makes no sense to support 10.9 because Cartography is already targeted to iOS 8.0 onwards which supports this new API. This should "fix" #274.

• Extending support to NSLayoutGuide: A missing feature in the release of Cartography 3.0 was the support of NSLayoutGuides in macOS. This is now supported!

• Improvements to insetting edges: I added a more expressive way to inset edges in expressions. The way you would currently do this would be expressing view.edges == inset(view.superview!.edges, 10) which would now can also be expressed by view.edges == view.superview!.edges.inseted(by: 10) which makes a lot more expressive.

• Supporting other layout guides included in UIView: UIView also includes references to other layout guides that are used to define standard layout margins or readable content margins. These now can be used in Cartography by expressing for example: view.edges == view.superview!.readableContentGuide.edges

[corujautx]

@orta could you please check this? Thanks!

[orta]

Yeah, this looks good to me - thanks for the comprehensive PR text too - will ship a release now

[orta]