Support for netstandard 2.0

With this release, System.Extensions adds the netstandard2.0 target to its nuget package so you can now start to build .net standard libraries on top of it and use it in .net core applications (#14).

As usual, this release can be found on nuget: v2.0.59.

First github release

Breaking changes compared to 1.x:

• IFilesystem no longer offers async methods, instead every method is synchronous #12

Small improvements:

• Introduce IFilesystem.WriteAllText, ReadAllText and similar methods #13