Align with HttpClient multi-targeting recommendations

[richardszalay]

Based on: https://leastprivilege.com/2018/05/21/the-state-of-httpclient-and-net-multi-targeting/

Will require the following changes:

• Add a .NetStandard2.0 target that only references NETStandard.Library
• Update the .NETFramework4.5 target so that it has a frameworkAssembly rather than a dependency

The rest of the targets can remain as-is for compatibility, but since this will change the behavior for netfx45 we'll need to release it as a major/breaking version.

[YehudahA]

#36

[richardszalay]

Indeed! If only they'd published their recommendations sooner.

[richardszalay]

Released in 5.0.0