Add "report only" option

[amosonn]

Fixes #17, replacing #21 .

I addressed some of the issues mentioned in the original PR; the main one, regarding the thumbnails, remains open - I'm not sure what is the right way to pass the flag down to jpg_check_structure. Adding it into file_recovery_t seems wrong and too invasive; the alternative being adding another argument, or passing params all the way down?

Since, for the time being, thumbnails are still being created, I didn't want to remove the line creating new directories, since they might be overcrowded by the thumbnails otherwise.

[jmiguelf]

so, this 'report only' option is nice, but it still creates the file and dirs...