Add `on_prejoinplayer' callback.

Adds the ability for mods to register callbacks called before a player is initialized.

This can be used for customized ban managers (for example, ban by regex match, IP range, wordlist, etc), or simply to deny guest players from entering your server and cluttering up your world's `players` directory. There may be other uses as well, such as logging or whatever.

Simple example: http://pastebin.com/akMiaR6e