added option to automatically generate nuget dependency list from target...

...ed packages.  This is reflected in the NuBuildAutoGenerateDependencies property being set to true.

If NuBuildAutoGenerateDependencies = true (default) but a dependency list is specified in the nuspec file, the nuspec file wins, and no auto-generation is done.
Also updated Nuget.Core to 2.8, and pushed the version to 1.9.1