currently the renew function does always return 0 even when the cert issuing failed. for now just return the function return code of issue().
maybe an additional offset (like +100) should be added to distinguish the issue error codes from the renew error codes.
1. separate installcronjob/uninstallcronjob. no-root users can use cron job without installing le.sh
2. add cron command for cron only.
3. polish help messages.
4. move le from /bin/le to /usr/local/bin/le
5. only root can install to /usr/local/bin/le. non-root users can use ether /usr/local/bin/le or ~/.le/le.sh instead
6. WORKING_DIR can be specified when install/cronjob