implement --backup / --restore

Sometimes it is really convenient to create an archive of an analysis so you can run an  analysis workflow with the confidence that, no matter what, you have something to fall back on. `workflow --backup` should prepare a bzipped archive in the `.workflow/` directory with a filename like `.workflow/backups/YYYY-MM-DD-HHMMSS.tar.bz2`.

Might also be nice to give the user the ability to `--restore`, or something along those lines, to allow the user to restore the analysis back to the state on such-and-such a date. 

Perhaps this should be tied to a repository hash id for `git`/`hg` to make it easier to restore an analysis to a particular state during the development pipeline?