use lumberjack for log rolling

This is very similar to the last PR, except I didn't change out the upstart config stuff... since the output redirect doesn't hurt anything, and we need the rest of the code to properly set permissions and owner on the logfile.
Or you could save 2-3 backups of the log. 100 MB is not a lot. Given the amount of logging juju does, I would raise this to about 300 MB an keep at least 2 backups.