Abstract $bb_current_user phase 1

[mdawaffe]

Plugins which modify bbPress pluggable user functions can be simpler if we don't rely on $bb_current_user as directly as we do.

[670] introduces bb_get_current_user_info(). Let's use that and bb_is_user_logged_in() as much as possible.

In phase 1, we will still rely on the the BB_User methods and meta data directly from the $bb_current_user variable.

(In [671]) Reduce dependence on $bb_current_user global phase 1. Fixes #567

(In [672]) typos. fixes #567