make le more friendly to no-root user.

make le more friendly to no-root user.
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