Adds ApplicationHelper#current_user to find user by session token
Mixes in ApplicationHelper into ApplicationController
Creates User::generate_token, #reset_session_token!; updates User#ensure_session_token to use User::generate_token
Adds ApplicationController#logout! and SessionsController#destroy
Adds a logged_in? function
Fixes typo in SessionsController#destroy
Conditionally displays User actions in Application layout