Use YARD for documentation; add to Rake
Add Yardstick and tasks.
Yardstick: Add @api tags.
Some more yardsticking; refactor privilege_set.
Split privilege_check from privilege_set.

Also do some YARDing.
YARD up privilege_set.
YARDify privilege_subject.
YARD up PasswordCheck too.
100% YARD coverage.
Make rubocop happy too.