Minor update to installsu.sh

The old version worked but i coded it sloppy. I fixed duplicate calls to busybox (ex: '/system/bin/busybox busybox su' is now just 'busybox su'). Also, It is much easier to follow if it ever needed to be modified in the future.
Merge pull request #20 from jjfriede/patch-3

Minor update to installsu.sh