getPDFSignatureCount: fix "too much recursion" error with some PDFs

The regex used to detect signatures uses "(.|\\s)" to emulate dotall, since
old browsers don't support it.

For some reason that leads to internal errors in Firefox with larger files in some cases.

Replacing it with a simpler "[\\s\\S]" class makes things work.

Also adds some tests with real PDF files, "QPDF-367-0.pdf" was triggering this issue.

Fixes #46