Added changes per Jordan's code review.  Now there's an admin form
defining variables for PDF derivative creation.  All other
derivative creation is handled by either large image or ocr.
Added an install hook to make sure admin variables exist.
Replaced &= with long form &&
Filename conventions.
Merge pull request #3 from daniel-dgi/7.x

7.x
Merge pull request #4 from nigelgbanks/7.x-filename-conventions

Filename conventions.