Add .NET 4 project and solution

This will facilitate building a .NET 4 version of the application for
NuGet distribution.

i'm never figured out the best way to pack NuGets again multiple versions of .NET. This bit of code is inspired by my (mis)understanding of JSON.NET's NuGet packing process for multiple frameworks.

Is this something you want to add? If so, i can build a nuget packing powershell script for you. If not, i'll just use my own fork of this library until i can upgrade my parent application to .NET 4.5.