Add netstandard2.0 target

[mysticmind]

• Add .Net Standard 2.0 target
• Miscellaneous cleanup of csproj files
• Remove global.json file
• Recreate solution file which was having old xproj references

[mysticmind]

@jeremydmiller are you planning to look at this PR and merge it?

[jeremydmiller]

Sorry I was slow. I'd lost sight of this. I'll get a nuget out soon.