Addressed code cleanup using R#.
Addressing R# code issues

This commit introduced null instead of null object variable.
Refactoring SpecimenDefaultValueCommand
Cleanup msbuild scripts using R#
Enable to restore nuget packages
Deleting all the nuget packages
Updating the gitignore file

The '/' character causes confusing between full path and certain folder
name. So if ignore path has  '/' , it should be considered full path.

Not to include nuget packages, the packages should be excluded in git
repository and be ignored. However, the repositories.config file is
needed to find packages used on solution level.