It appears that the switch to bcrypt causes problem on a server.

- The server has a fairly old PHP version running, which doesn't support
  our bcrypt compat library from https://github.com/ircmaxell/password_compat
- To mitigate this issue, we check if the provided hash function works,
  and fallback to md5 if necessary.
- It might be necessary to reopen issue #7, and to introduce a better workaround,
  but I'd like a newer PHP version on that server better.