disable mysql automatic startup on Ubuntu 13.04

Ubuntu has abandoned SysV init script approach to booting the server since 10.04.This link explains
So to prevent mysql from starting on boot, you have to switch to a new way as belew:

1. go to the /etc/init directory
2. open the mysql.conf file
3. comment out the start on line near the top of the file, this might be spread across two lines, so it is better to comment out both.