Help Message for Machine Readable Output in `cvmfs_swissknife scrub`

This prints a table of all possible error and warning messages that `cvmfs_swissknife scrub -m` might emit. It is printed for `cvmfs_swissknife scrub -h`, thus this Pull Request adds a *HelpSwitch* feature to the parameter handling of `cvmfs_swissknife`. A *HelpSwitch* can override any other mandatory parameter checking.

*Note:* This contains the (open) Pull Requests [here](https://github.com/cvmfs/cvmfs/pull/381) and [here](https://github.com/cvmfs/cvmfs/pull/382).