netcore (dotnet) 2.0 support

[James E. King, III]

Hi folks,

I converted the netcore library over to netstandard20 and netcoreapp20 and
used the official dotnet 2.0.0 SDK on linux.  The netcore library also runs
as a client (but not a server yet) in the cross test.  I put up the pull
request 8 days ago but I haven't seen any responses to it.  I'll give it a
few more days, and then I'm planning on merging it unless I hear otherwise.

Thanks,

Jim