Add support for JSIL as a Protobuild module

I've added project definitions for Protobuild to JSIL.  Protobuild is a cross-platform project generator for C#.  More information is available at https://github.com/hach-que/Protobuild.

Protobuild is supporting a Web platform by implicitly / transparently using the JSIL compiler for executable projects.  This is one of the first steps to bringing full JSIL support to MonoGame (which now uses Protobuild).

Ideally it'd be good to replace the existing C# projects and solutions with Protobuild, but I'm aware that there's projects and solutions (such as the website project) which aren't directly related to using JSIL as a library (and as such I haven't ported them).