Closes #17 maintenance mode status code

Now sends 503 HTTP code and Retry-After of 24 hours.

http://googlewebmastercentral.blogspot.co.uk/2011/01/how-to-deal-with-planned-site-downtime.html

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.37