You need, in your $PATH:
 - unpaper
 - ocropus (this drags along a horrible set of dependencies, sorry. A future project is extracting the tiny part scandal actually uses.)
 - ImageMagick (in particular, the convert utility)
 - pdf2dsc, part of ghostscript (which you'll also need to allow imagemagick to process pdfs)
 - a standard saneish linux/bsd environment (both Linux and OS X work)
 - (probably) bash - I haven't made or checked its compatible yet.