Simplify persistence cookie security and bump version to g5.

Now it's more or less a simple password mechanism, which isn't very secure, but doesn't have any of the flaws that the old solution has. Additionally:
- Cookies are set securely (if HTTPS is enabled) and with the HttpOnly flag.
- Persistence cookies can be disabled completely with the esoTalk.disablePersistenceCookies config option.

Fixes #30 (finally!)