Extend hooks implementation

This relates to: http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=41897

As an alternative to reimplementing a complete new hook system for phpBB 3.1, implement the requested hooks (http://area51.phpbb.com/phpBB/viewforum.php?f=111) with the "Olympus hooks". If may turns out that some hooks aren't able to being implemented with this hook system but that can be discussed in the topic relating to the hook.