Use "Assert.That" consistently to be more NUnit4-ready

Use "Assert.That" consistently to be more NUnit4-ready

Fix target runtime to 2.1 to align with SDK

Fix version of signed assembly

Add SourceLink support for easier debugging

For more information, see:

- https://github.com/dotnet/sourcelink
- https://blog.nuget.org/20180827/Introducing-Source-Code-Link-for-NuGet-packages.html

Bump version to 3.3

3.2 release

Reference NCrontab.Signed.nuspec in VS solution file

PCL version

Nuget package release notes