add FACEBOOK_APP_ID and FACEBOOK_SECRET into .bashrc, source ~/.bashrc, restart service and visit localhost:3000/auth/facebook to check if the facebook setting is right
add some routes, to identify user login status
add session controller
generate user model
finish from_omniauth method which store user facebook id info into user model
build current user method in both controller and helper
add login link in the nav bar
use js to do login and out via ajax