Documentation Project

This is the first commit of the documentation project for MonoGame.

 - Uses [SharpDoc](https://github.com/xoofx/SharpDoc) as the generator.
 - We build class reference for both the framework and the new pipeline.
 - Generator currently runs for windx, wingl, win8store, wp8, linux, and web.
 - Copied over existing documentation content from website.
 - Updated version of Protobuild which supports `<DocumentationFile>`.
 - Hooked it up to the build script.

This will be followed by hooking in the rest of the platforms, style sheet improvements, and more content.