Implemented better password hashing

- We now use bcrypt with salted hashes.
- There's no change to the database layout, as I've anticipated different hashes.
  I just was lazy originaly where I shouldn't have been.
- For compatibility with php version <= 5.5 we use a compatibility library from https://github.com/ircmaxell/password_compat
- We now follow the best practises for this problem as mentioned in http://php.net/manual/en/faq.passwords.php
- Old md5 hashes still work for the first login, but will be replaced automagically on login.
- Fixes #7