Bump Xamarin Studio to v4.2.3
Add Xamarin.iOS at v7.2.1.25
Add Xamarin.Android at v4.12.3-3
Bump Mono MDK to v3.4.0.185
Merge pull request #3745 from jamesottaway/master

Xamarin Studio, incl. Xamarin.Android, Xamarin.iOS and Mono MDK