Auto Mount USB drive When it is plugged in.

https://www.raspberrypi.org/forums/viewtopic.php?f=66&t=132926

Jessie is refusing to just mount the fstab properly, but this 
effectively just recreates the manual fix which is to umount the 
USB drive and then mount it again - but as a startup action

sleep 5
umount /dev/sda1
sleep 20
sudo mount -a