Adds the ability to set a unique hostname

Hi Alex,

**TL;DR This pull request adds the ability to set a unique hostname during initial set-up.**

This will be a huge help in classroom and hack space environments where many new images are on the same LAN at once. 

Without a monitor, it's hard work locating your freshly configured RPi on the network for SSH access when they all have the same name... many users want to remote in with a laptop. 

Alternatives such as the serial console to find your IP are a bit overwhelming for beginners. 
As is mounting the SD on a laptop and hunting about in /etc

I have tested this on my Model B and hope you see fit to include this contribution. 

Please let me know if you need me to modify anything. 

Kind regards,
Andrew Stone.