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These have been replaced by the Xamarin Cask in homebrew-cask.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.
Merge pull request #307 from fapper/xamarin-products

Add Casks XamarinAndroid, XamarinIos, and XamarinStudio