Protobuild

This pull request starts to add Protobuild (https://github.com/hach-que/Protobuild) support to MonoGame. The work is based on the work @hach-que did in PR #1945.

 - Removed old MonoGame.Framework solutions/assemblies.
 - Added Protobuild binary and definitions.
 - Fixed build script to trigger Protobuild.
 - Fixed installer to reference changed assembly output paths.

To generate solutions/projects you simply run Protobuild and it generates everything.  For people running their own build servers you will have to add a step to do that.

The next steps after this is merged to convert the tools (MGCB and 2MGFX) and then the content pipeline tools as Protobuild definitions.