- bugfix: noParallel() didn't remove 'parallel' from validParOptions
  allowing it to sneak through to the Pstream layer.
  noParallel() now clears the entire validParOptions as well

- new convenience methods
  * addOption()
  * removeOption()
  * addBoolOption() - as per addOption(), but for bool options (no param)

- printUsage() output format
  * options sorted alphabetically
  * options listed on separate lines for better readability

- new optionUsage static member for short usage information about
  an option
  * corresponding addUsage() method or provide usage information
    in addOption() / addBoolOption()