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.
I apologize for the vanity :)  Retyping for those who weren't on IRC... this is my personal code, written in off-hours, and thus not owned by Canonical.  If it had been written during hours, it would be owned by Canonical and therefore should absolutely be under github.com/juju.  If there comes a time when the needs of Juju diverge from what I'm willing to put into my personal copy of Lumberjack, then we can fork the project under github.com/juju and adjust it as necessary.  

For now, I think it's a good policy to give credit to third party packages by using their standard URL when possible.