added Nuget packager for v3.1.1

I created a nuget folder which contains the proper files for use with NuGet (ASP.NET package manager). It will pull the exact files from the appropriate folders within the master branch. Only oddity is the version number in the Package.nuspec file. I am waiting on a response from the NuGet team, but I accidentally published a package with version 3.1.1 and unfortunately they don't allow you to overwrite a version. So instead I had to make it 3.1.02, which is of course not ideal.