This commit adds convenience methods to repair PDFs on the fly and/or to
write repaired PDFs to disk.
Currently, this does so via Ghostscript using the method we've asked
many users to try by following the instructions at
https://superuser.com/questions/278562/how-can-i-fix-repair-a-corrupted-pdf-file
Now, hopefully, this saves folks a few steps.
Jeremy Singer-Vine