Hooks

bah. also includes the timezone stuff, thanks to my fucking up my branches.

hook for auth to modify user/pass (if required)

hook for load/save user prefs, can optionally bypass loading/saving from db and in the case of load will merge prefs loaded from db with the ones loaded via the hook (in case someone wants to keep something like timezone in an external config store common to the rest of their apps and the agendav-unique preferences only in the db)