[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140814_do_not_choke_on_broken_filenames'

Filenames sometimes contain invalid UTF-8 characters which triggered
encoding exceptions causing the inspection to fail. We now filter those
files and show a warning to the user about the skipped files instead.

Fixes #59

**Note:** This implementation uses the [string-scrub](https://github.com/hsbt/string-scrub) gem, so we need to put that package into the obs before merging this pr. I have prepared the package in our devel project already.