Replace password hashing with the new PHP Password Hashing API

Passwords are stored as Bcrypt-hashes instead of SHA-1-hashes. This is completely backwards-compatible, old password-hashes will be replaced with new hashes when the user logs in.

Read more about why Bcrypt is safer than (salted) SHA-1 here: http://codahale.com/how-to-safely-store-a-password/

PHP 5.5 Password Hashing API: http://php.net/manual/en/book.password.php (compatibility library: https://packagist.org/packages/ircmaxell/password-compat)