Add netstandard2.0 as a target

[petrsvihlik]

Problem:
When installing the SDK, user can sometimes get this:

There are too many package dependencies. We try to reduce the number of references to the necessary minimum.

The root cause is actually targeting netstandard1.3: https://stackoverflow.com/questions/47365136/why-does-my-net-standard-nuget-package-trigger-so-many-dependencies. In CM Helpers SDK we're using netstandard2.0 + net45 targets and there's no such issue.

Here's a recommendation from David Fowler:
https://twitter.com/davidfowl/status/1008453902058917888

I know we said target The lowest netstandard but we really want everyone to re-baseline at netstandard 2.0. At the very least add a netstandard 2.0 target.

Other services use the same approach as well.

Solution:

• add netstandard2.0 target to reduce the dependency footprint of the SDK.

Additional steps we can take to reduce the dependency footprint are described in #117.