Currently the Command and CommandResult classes are mixed into the nix
fixture file.
This commit moves them out into their own lib file, to make it more
obvious that they can be used standalone for other applications too
Additionally improved documentation of said classes
and bumped log level of stdout and err on unexpected exitcodes, as it is
within an error context
Change-Id: If2d554acde86fd54f2445fc46453f06923af5fe9
Commentator2.0
438cb4cb31