Migrate to netstandard 2.0

[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

If anyone needs this package now, please use my forked nuget: https://www.nuget.org/packages/Fusonic.Ical.Net/5.0.2

[rianjs]

This was a great PR, thank you.

It's been updated in nuget as well: https://www.nuget.org/packages/Ical.Net/4.2.0

[rianjs]

I invited you to be an admin/collaborator.