Support net standard (v1.1)

[danzel]

This includes and superseeds #7

Update the package to target NET Standard 1.1 so it can be used by normal .net framework and .net core projects.

This PR drops the targetted version of standard to 1.1 so the package can be used by net framework 4.5 (previous PR was 1.4, so .net 4.6 was required)

[msftclas]

All CLA requirements met.

[danzel]

@huxuan It'd be cool to get this merged and nuget'd, then I can send the same for the Face API to fix microsoft/Cognitive-Face-Windows#39

[huxuan]

@danzel I am not the project owner for this, but I will ping some guys that might be able to handle this.

[danzel]

Ah sorry for the ping then. Thanks :)

[cthrash]

@chsienki - can you give me a hand on this? I'm not quite sure what this change means for, say, UWP. Also, are we possibly going to break Xamarin projects? Currently the supported profile is Profile259, which means .NET Framework 4.5, Windows 8, Windows Phone 8.1, Windows Phone Silverlight 8. Is .NET core a proper lowest-common denominator?

[chsienki]

netstandard1.1 supports everything but Windows Phone Silverlight 8. (See https://docs.microsoft.com/en-us/dotnet/standard/net-standard)

I think at this point its probably fair to say we no longer support WP8.0, but this is technically a breaking change, which would require upping the package version to 2.0.

As long as we change the version number I don't have any problems with it, as downstream packages will then have to opt-in to the netstandard version, and any others won't be affected. We probably want to bump the major version on the down streams too, as they pull in support, and ripple the netstandard update across them.