bug 848680 - add pig support to crontabber

I perhaps named this branch badly. Basically, it's a replacement of https://github.com/mozilla/socorro/blob/master/scripts/crons/cron_modulelist.sh as a crontabber app. 

The diff is large because with this it adds a `SubprocessMixin` class which is handy for all work with command line things. I like its API. `exit_code, out, err = self.run_process(command, input=None)`

I have NOT tested this manually. I don't have `pig` or `hadoop` installed and I guess I could make fake programs but that faking would just be as solid as the unit tests. 

Please scrutinize the way the backfill date is converted to yesterdays date and inserted into the various commands.