Support .NET Standard 2.0

[krdmllr]

Hi,

this PR brings all required changes to support .NET Standard 2.0

• Use new SDK style project file
• Target .netstandard2.0 besides .NET 4.6.1 for legacy support
• Remove System.Drawing.Common since it depends on GDI which can lead to problems (for example in Azure Functions as I experienced :P ) -> Replaced with SixLabors.Fonts (Apache-2.0)
• Remove nunit dll's and use NuGet package instead

Additionaly package information could get added to the new project file as a .nuspec file for packing and publishing the package is obsolete https://docs.microsoft.com/de-de/dotnet/core/tools/csproj#nuget-metadata-properties

[thomasjo]

Thanks for your contribution @krdmllr 🥳 👏