Fix consumption of package for .NET Core and .NET 5
Fix consumption of package for .NET Core and .NET 5

Fixes #38

`netcore` is not a valid `targetFramework` value, so was being ignored by NuGet during package resolve.

Using `netstandard1.0` instead will ensure the package's files are referenced by the project correctly in .NET Core and .NET 5 projects.

jingwood/d2dlib,jingwood/d2dlib,jingwood/d2dlib