Updated to MonoGame 3.6 + Directory refactor

- Barotrauma's projects are in the Barotrauma directory
- All libraries are in the Libraries directory
- MonoGame is now managed by NuGet, rather than referenced from the installed files (TODO: consider using PCL for easier cross-platform development?)
- NuGet libraries are not included in the repo, as getting the latest versions automatically should be preferred
- Removed Content/effects.mgfx as it didn't seem to be used anywhere
- Removed some references to Subsurface directory
- Renamed Launcher2 to Launcher

Libraries/NuGet/.gitignore

@@ -0,0 +1 @@
+/*/

Libraries/NuGet/readme.txt

@@ -0,0 +1,4 @@
+Not including NuGet packages in the repository, Visual Studio
+should download them automatically when you build the solution.
+You can also manually restore them all by going into the package
+manager and clicking "Restore" when prompted.