Added dns pre-fetching reminder link into header

Also added preventative prefetch control to stop some browsers (chrome) prefetching all URLs on the page.

See:
- https://developer.mozilla.org/en-US/docs/Controlling_DNS_prefetching
- http://www.buyog.com/code/?id=53
- http://davidwalsh.name/dns-prefetching
- http://dev.chromium.org/developers/design-documents/dns-prefetching