Add support for .NET Framework 4.7.2

[joelpj]

I found useful your library, but my project requires .NET Framework 4.7.2 and the available NuGet package doesn't include support for this platform.

I copied your code as it with added LGPL license header and the appropriate attributions while a supported version will be available.

Also would be better it was licensed with a permissive license.

[jasonterando]

Hi, thanks and feel free to use however you'd like. I'm going to switch the license to MIT and update the package to use the latest AWS libs. I wrote this on Linux but if getting to work on 4.7.2 is easy I'd be happy to do it (maybe just adding an entry to TargetFrameworks?)