implement sign-in and sign-out

Changes:
* move links for signed in users to dropdown menu
* show flash message on successful signin
* add spec for signin upon signup
* add spec user stays logged in
* add sign out
* signin upon signup
* add signout and profile links for signed in users
* implement session helpers for signin
* add spec and impl for remember_token
* add remember_token to user
* include sessions_helper in all controllers
* add user signin to sessions_controller
* add test for flash message persisting too long
* implement invalid signin behavior
* add tests for signin page
* add signin controller and stub signin view