Add a Machine Readable Output Mode for cvmfs_swissknife scrub

This adds the flag `-m` to `cvmfs_swissknife scrub` which produces a machine readable data integrity report. It should allow cron jobs to make use of the outputs of `cvmfs_swissknife scrub`. Output looks roughly like this:
```
[alert ID] [affected path] [affected hash]
```

Additionally this introduces a help switch `cvmfs_swissknife scrub -h` which prints some documentation information, about what each `alert ID` means.

*Note:* This Pull Request depends on [this](https://github.com/cvmfs/cvmfs/pull/382) and [this](https://github.com/cvmfs/cvmfs/pull/381).