Add netstandard support to appropriate packages

[Eilon]

The Web API "client" packages should have support for the lowest netstandard TFM possible.

[dougbu]

Blocked: Have a branch ready to go but #32 blocks applying it.

Can't create a true .NET Standard library without more-recent tooling. Also unable to avoid Microsoft.Net.Http package and its "unique" System.Net.Http.Primitives.dll dependency when building a Profile259 library.

[dougbu]

Unblocked: A couple of the CI machines have VS 2015 installed and they should be up-to-date enough to build these new projects.

[dougbu]

Git portion done in f830808. Will wait for a build on CI then do TFS portion…

[dougbu]

Nightly build was successful after both checkins:

• Microsoft.AspNet.WebApi.Client.5.2.4-alpha1-161213.nupkg published to https://www.myget.org/F/aspnetwebstacknightly/api/v2/package/Microsoft.AspNet.WebApi.Client/5.2.4-alpha1-161213 and visible in gallery at https://www.myget.org/feed/aspnetwebstacknightly/package/nuget/Microsoft.AspNet.WebApi.Client
• Microsoft.AspNet.WebApi.Client.5.2.4-alpha1-161213.symbols.nupkg published to http://nuget.gw.symbolsource.org/MyGet/aspnetwebstacknightly
• Confirmed latest package works fine with a hacked Microsoft.AspNetCore.Mvc.WebApiCompatShim project in the aspnet/Mvc repo