Add --clean to reset all repos to upstream master

prep_source_repos already does a great job of applying a series of
patches - but sometimes you just want to temporarily set all the repos
back to their upstream state just to check how that's working today.

This change adds a --clean option which resets all the repos back to
upstream master state.
Merge branch 'clean'
Enable use of local reference directories

*If* you already have on-disk local copies of the repos you're cloning,
using them as a reference can be a significant time and bandwidth
saving.

If you don't already have them, then *shrug* you're gonna have to use
the network. If your local repo is stale, I'm told git is Teh Clever and
will pull the missing refs from the network.
Merge branch 'use-reference' into jp-master
Update .gitignore to ignore virtualenv cruft
Add basic requirements
Basic packageization.

Readme has been tweaked to reflect the fact that this is now a more
generic tool.