Requests for comment/Extension registration

Some brainstorming about what information extensions need to "register".


 * classes - $wgAutoloadClasses
 * hooks - $wgHooks
 * API modules - few globals for this
 * special pages - $wgSpecialPages
 * config options
 * namespaces, NS_CONSTANTs - this is a mess
 * sql tables
 * RL modules
 * userrights / permissions
 * ContentHandler stuff
 * $wgExtensionFunctions
 * credits - $wgExtensionCredits
 * version number - $wgExtensionCredits
 * dependencies - not currently supported

Ideally most of this is stored in a JSON file or something, so it can be read without require_once'ing the file.