Windows Nano Server support

[alexpilotti]

This module does not work on Nano Server due .Net's "System.Net.WebClient" not being available in the CoreCLR.

"System.Net.Http.HttpClient" is a viable alternative as used for example here:
https://github.com/cloudbase/unattended-setup-scripts/blob/master/FastWebRequest.psm1

[ferventcoder]

I like the start of that, but it should fall back to System.Net.WebClient when that is available - the code you have is only compatible with .NET 4.5+ (I believe) and yes, CoreCLR.