Add Carthage support

[basememara]

You can test by adding my forked repo into your Cart.file:
github "basememara/AlertOnboarding"

[basememara]

The only thing I don't like is the namespace for the framework: import AlertOnboarding_iOS

For usingimport AlertOnboarding, the project has to be recreated so that the example app is not using the desired namespace. I can help if you like: http://basememara.com/creating-cross-platform-swift-frameworks-ios-watchos-tvos-via-carthage-cocoapods/

[PhilippeBoisney]

Thank you for your pull request ! I don't have the time now for check it, but I will test and approve it as soon as I can.