Handle changed password storage

This depends on an API pull request: https://github.com/joindin/joindin-api/pull/104

Having changed the format of the password column in the database (it's now a password_hash()ed md5 of the password), the old website needs to know about that.  This makes sure that the new password format is used when:

 - registering an account
 - requesting a new password
 - changing your password by editing your account
 - logging in
 - using the v1 API

Did I miss anything?  This feature uses the password_hash/password_verify features from PHP 5.5 so if you're still using the dev platform that runs 5.3, you'll need to grab https://github.com/ircmaxell/password_compat and require it at the top of index.php