Automatically unmount/detach backup drive after `backup` script completes

[necolas]

Some reference docs:

• https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/hdiutil.1.html
• http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man2/unmount.2.html

Need to work out what the correct and safest way to do this is.