Migrate to netstandard 2.0 and nodatime 3 + cleanup

[davidroth]

This version has been cleaned and upgraded to netstandard 2.0 Changes compared to official package:

• Target netstandard 2.0 / netfx472
• Use netcoreapp 3.1 for tests
• Upgrade noda time package to latest version (3.0)
• Removed unusued references
• Removed duplicate FrameworkUnitTests

If anyone needs this package now, please use my forked nuget: https://www.nuget.org/packages/Fusonic.Ical.Net/5.0.3

[rianjs]

Just making notes to myself:

• Figure out what the new ical version number should be, and change it
• Fix PackageId in the csproj
• Figure out why I had Framework-specific unit tests -- there was a reason