Enable Strong Naming for Cirrious.MvvmCross and Cirrious.CrossCore

This is a pull request for [Issue 232](https://github.com/slodge/MvvmCross/issues/232) about strong naming.

Ideally, the Key.snk shouldn't be in the public repository, and should be kept somewhere that's only used during the official build process, but having a strongly named assembly with the key being public isn't really that different from having non-strongly named assemblies.

I only did the 2 key projects that would typically be used in a ViewModel/Core kind of setting.  The .csproj change can be copied and pasted into any of the other projects (or all) as needed.