implement authbackend to authenticate existing users

- add .coverage to .gitignore
- implement caffeine.authbackend.LegacyCoffeestatsAuth to login existing users
  and upgrade their passwords on first successful login
- implement caffeine.tests.LegacyCoffeestatsAuthTest to verify
  LegacyCoffeestatsAuth's behaviour
- add passlib and py-bcrypt to requirements/base.txt for checking bcrypt
  encrypted passwords