Implements hooks, an easier way to write code to be called at startup/roundstart/roundend/etc.

Current hooks are "startup", "roundstart", and "roundend".
Most stuff in world/New() has been moved over to the startup hook.
Roundstart and roundend have no hooks yet.

Removed the unused "newbanjob.dm" file and associated verbs/topics.
Bumped RECOMMENDED_VERSION up to 501.