[*] Security : Use a better password hashing method

This changes the password hashing method to use phpass-0.3 instead of md5 + a install-wide hash. I hope using phpass is fine for you (it's public domain, and is used in Wordpress and Drupal last time I checked).

It handles password upgrades the first the user logs in again though, because we obviously don't have access to the plaintext.

Notes :
- [ ] I didn't do anything w.r.t the database structure update (I did those manually), but I'm expecting there's an automated script somewhere that must be updated because the column size changed.