Fix multiple-character handles

[z38]

This PR implements a non-BC change to support older versions (fixes #1 and replaces #4). With this fix, newer versions can handle up to 702 files whereas users with an older version of pdftk are limited to 26 files.

Generated file handles: A, B, ..., Z, AA, AB, ..., ZZ

We should probably add a small note in the code or README about the fact that older versions of pdftk are unable to handle many files.