Add Casks XamarinAndroid, XamarinIos, and XamarinStudio

These have been replaced by the Xamarin Cask in homebrew-cask. They are locked at the following versions, as Xamarin has bundled all three in a new, universal installer retrieved by the Xamarin Cask added in pull request caskroom/homebrew-cask#5118.

I'm not exactly sure what the version naming convention is for this repository please comment if you think the Casks should be named differently. It might be a good idea to note down some conventions.