Allow building a non-PCL and put PCL and non-PCL in nupkg.

Manually edited the .csproj to allow building C5 as a PCL and a non-PCL and wrote a .bat script to build a nuget package containing both the PCL version and a net40 version. PCLs do not work on versions of Mono earlier than 3.x. Even the latest version of Ubuntu is not on 3.x yet.

I'm not sure how Visual Studio treats the changes in the .csproj or if it will even open the project. I only have the express edition which does not open PCL projects. Command line builds do work though.