PlatformTarget update to .csproj files

On any Windows machine with .NET Framework 4.5 installed, there is a
compiler warning regarding mismatch between processor architecture being
built "MSIL" and the processor architecture of the reference
"Microsoft.Xna.Framework.*"

Modified project files to resolve this warning, per MSDN blog:

http://blogs.msdn.com/b/astebner/archive/2012/05/03/10300809.aspx