Use factor-bundle

[RichardLitt]

It would be better if we could use factor-bundle to watchify the background and content script separately, as well as possibly any script that is native to the PDFViewer. This would watch all of them, and bundle up a common.min.js file of code that could be shared between them.

Currently, factor bundle doesn't work, for issues noted in browserify/factor-bundle#61. I've got no idea how to diagnose that bug. I've got code for it (old now, probably), on ec01716