add support for net40 and net45

[moh-hassan]

Rebased from #387

• Add support for net40 and net45 for the netstandard2.0 project.
• Resolve the null EntryAssembly Problem in issues: Version 2.4.x System.ArgumentNullException parsing options in unit-test #389 forever.
• Pass the test case 'Add unit tests for Issue389 #[392]'([no-merge] Add unit tests for Issue389 #392)
• Create commandline multi-target project net40;net45;netcoreapp2.0 .
• Create commandline Test multi-target project net461;netcoreapp2.0 .
• Add class to support net40
• Modify the cs demo to run in net40;net45;netcoreapp2.0;netcoreapp2.1
• Pass all tests (334 test case) in both net461 and netcoreapp2.0.
• Appveyor CI success on the fork project:

[moh-hassan]

@ericnewton76