Remove SnapKit dependency

[fpg1503]

This PR removes the SnapKit dependency from Mandoline. It's still being used in the example project. I've created a small extension to make creating constraints a little bit more readable.

[anatg]

Hey! Thanks for opening this! Looks like there's some interest in removing the SnapKit dependency. I'm thinking that rather than creating another extension for AutoLayout, we might be able to use LayoutAnchors if we remove support for iOS 8. Going to work on that in another branch and update accordingly.