Rebase on .NET Standard 2.0

[danielgindi]

I know that iText 5 is EOL, but for people who are trying to migrate to newer technologies, it becomes very complex do it all at once.
Allowing itextsharp to compile for .NET Standard removes this obstacle from the way.

This is a rebase of https://github.com/itext/itextsharp/pull/39 on the latest.

[iText-CI]

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