Exchange PDFMerge for Ghostscript

[jackjennings]

The gs command should be pre-installed on linux, and give us a good utility for merging PDFs without the overhead or bugs that are part of the PDFMerge library.

https://www.linux.com/news/putting-together-pdf-files
http://stackoverflow.com/questions/8158584/ghostscript-to-merge-pdfs-compresses-the-result

This isn't a crucial change, but would be nice to shed the dependency on this library and shell out to Ghostscript to do the actual merge. This would be a better foundation to start adding additional functionality like stamping pages with a timecode, or adding page numbers—which seems to be supported by GS.