.NET 4.0 Assemblies and Combined 4.5 and 4.0 NuGet Packages

This PR fixes issue #207.

It updates the project build files to automatically build both 4.5 and 4.0 binaries. The build scripts are also updated so that the NuGet packages include both 4.5 and 4.0 binaries. 

Tests currently only build and run for .NET 4.5 code, I can update these to support 4.0 as well if required.