X-Clacks-Overhead de Sixthhokage1

This is a fork of the GNU_Terry_Pratchett extension, fixed to support http-equiv meta tag detection (as long as only one X-Clacks-Overhead meta tag is used). Thanks to IgnoredAmbience's review of the original extension for the required fix of changing "Document" to "document" in gnu.js.

Original description
================
In tribute to the late Sir Terry Pratchett many websites are adding the message "GNU Terry Pratchett" into the html header "X-Clacks-Overhead". This plugin allows you to see when a website is using the header and read the message included. The message is also displayed visually in the semaphore style from the upcoming board game "Clacks" by Backspindle Games.

Concept: Richard Atha-Nicholls
Art: Amy Atha-Nicholls
Coding: Rob Grundy