add new switches to `cvmfs_swissknife scrub`

Conflicts:

	cvmfs/swissknife_scrub.cc
add Alerts enum
replace PrintWarning by more flexible PrintAlert

Conflicts:

	cvmfs/swissknife_scrub.h
FIX: assure canonical path output
TEST: machine readable scrub output
Merge pull request #383 from reneme/feature-machine_readable_scrub_output

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.