Pipeline GUI Tool

This is the first commit of the Pipeline GUI tool for nice visual editing of the new MonoGame content pipeline command files.  The tool looks like this:

![pipeline](https://cloud.githubusercontent.com/assets/103522/2826714/104909b8-cf76-11e3-8045-b49b7210165f.png)

It currently has the following basic features which makes it usable immediately:

 - Loading, saving, and creating new pipeline project files.
 - Adding and removing existing content to the project.
 - Change importers and processors as well as its properties.
 - Build, rebuild, and clean content.
 - No dependence on Microsoft XNA or Visual Studio.

It was built using C# and WinForms specifically for Windows support.  We have a simplified MVC design intended to allow someone to eventually implement a new IView for MacOS and Linux.

Our next steps here are to get feedback from users, iterate on the core features, and improve the functionality.  A few things on our roadmap for the tool:

 - Editing custom processor assembly lists.
 - System for per-platform content/settings.
 - Support for linked content files.
 - Better property editors.
 - Undo/redo stack.
 - Actual help docs.
 - Recent file lists.
 - Proper unloading of processor assemblies.
 - Adding tool to MG installer and registering file extensions.
 - Triggering .mgcb building directly from Visual Studio build.
 - Import existing .contentproj files.

Thanks to @jamesford42 for making all the treeview and property grid work correctly.