Add Casks XamarinAndroid, XamarinIos, and XamarinStudio

Casks/xamarin-android.rb

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+class XamarinAndroid < Cask
+  version '4.12.3-3'
+  sha256 'e32095f71cd6bdfa190f3f1bc3a8b31d460c7f6845bfed6b851af10c1e0cfe58'
+
+  url 'http://download.xamarin.com/MonoforAndroid/Mac/mono-android-4.12.3-3.pkg'
+  homepage 'http://xamarin.com/android'
+
+  install 'mono-android-4.12.3-3.pkg'
+  uninstall :pkgutil => 'com.xamarin.android.pkg'
+end

Casks/xamarin-ios.rb

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+class XamarinIos < Cask
+  version '7.2.1.25'
+  sha256 'd95afc2fafd3e773809e18d39f1b830e0df172f39d3f60b0cfa55f4058eb19a2'
+
+  url 'http://download.xamarin.com/MonoTouch/Mac/monotouch-7.2.1.25.pkg'
+  homepage 'http://xamarin.com/ios'
+
+  install 'monotouch-7.2.1.25.pkg'
+  uninstall :pkgutil => 'com.xamarin.monotouch.pkg'
+end

Casks/xamarin-studio.rb

@@ -0,0 +1,9 @@
+class XamarinStudio < Cask
+  version '4.2.4'
+  sha256 'd5c2519822b134034d9d7ef6aad0c3311ca9fabc9d97b1069223125dca5ce1f1'
+
+  url 'http://download.xamarin.com/studio/Mac/XamarinStudio-4.2.4.35-0.dmg'
+  homepage 'http://xamarin.com/studio'
+
+  link 'Xamarin Studio.app'
+end