Switch to PowerShellStandard and NetStandard

[Jaykul]

I updated to .NET Core. Specifically, via the PowerShellStandard.Library (5.1.0). Can't use v3 of the PowerShellStandard.Library because it has a missing interface (and has never been finished). In any case, I don't think it actually matters which we reference, since that's just metadata, and everything we're using should be compatible, in the real world.

However, making it work in PowerShellStandard requires loosing Transactions, because they're not supported cross-platform. We could cross-compile and make a NuGet package with multiple versions in it, but it's probably just as easy to say: use the old version if you want transactions.

[powercode]

I would love to see this merged!