Evaluate necessity and feasibility of targeting netstandard 1.4/1.6

[masojus]

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After significant advancements are made toward wrapping up the Port Keen PCL to netstandard project, we should look at the benefit, utility and feasibility of pushing back to an earlier version of netstandard.

Reasons might included a client specifically needing to target an older version of .NET, UWP, Xamarin, etc. that doesn't support 2.0. Initial portability analyses for targeting 1.6 (iirc) flagged some problem APIs, but at least parts of that seemed like they wouldn't be hard to work around.

Another potential issue is that currently we have a binary targeting .NET 4.5, which only supports netstandard1.1 which might be hard to target, but also isn't likely worth it.

[masojus]

Some info on multi-targeting which specifically addresses a few of these potential scenarios: https://oren.codes/2017/01/04/multi-targeting-the-world-a-single-project-to-rule-them-all/