[Review] Request from 'aduffeck' @ 'SUSE/pennyworth/review_141002_cleanup'

Please review the following changes:
  * 90b48f1 Remove dead code
  * 49dd049 DRY up Cheetah exception handling
  * 1b8039c Merge pull request #13 from SUSE/fix_handling_of_optional_parameters
  * 5081bff Improve CLI error messages.
  * fe7959f Incorporate suggestions
  * 237b5cd Separate assignment from check
  * 6cd73bb Fix handling of optional arguments after GLI conversion
  * aa798d6 Allow optional arguments in method shift_arg
  * 1165ae6 Fix handling of optional arguments after GLI conversion