add Sessions controller with new action
add signin and signout routes
add rspec tests for signin_path
add new.html.erb for Sessions
add valid signin test
create signin form
add SessionsController#create action with flash msg
and add test for it
add remember_token field ot User model
add before_create callback which create remember_token from random string
and saves it as SHA1 hashdigest
create SessionsHelper#sign_in method
add SessionsHelper#current_user methods
use them to add 'Account' header menu item
signin on signup
create SessionsHelper#sign_out method
use it from SessionsController#destroy method to sign user out
add test it
add 'valid_signin' helper
add 'have_error_message' RSpec matcher
and use them in spec/requests/authentication_pages_spec.rb
ex. 1 done