Re-implementing the Remember Me feature

This re-implements the remember me feature in a simple way. This is a WIP.

* A unique key is stored in the #__users table in a new column. This key is stored in the cookie of the user.
* The user is recognized by the set cookie and by the value of the cookie, which corresponds with the key stored in the #__users table.
* The user is logged in based on the cookie. On login the cookie is created, on logout, the cookie is removed. If the cookie value is not found in the DB, it is removed again.

Issues:
* The old remember me feature has not been entirely removed yet.
* Is JUserHelper::genRandomPassword(32) secure enough to generate the key? What else could we use?
* The empty Remember Me Authentication plugin is ugly and it needs language files, etc.