DNX/.NET Core support

[adamhathcock]

I was just wondering your plans for DNX support. I'd like to get Akka.NET therefore this library on .NET core eventually. I started a branch for Helios here: https://github.com/helios-io/helios/compare/helios-2.0...adamhathcock:dnx?expand=1

I had to move to XUnit but everything else is pretty much the same. It also seems like Helios could move to xproj without any problems and still support .NET 4.5 as well as DNX45 and DNXCORE50