It's possible to use `OC_USER` as normal but the real stuff
is done by the `OC_USER::$_backend` class, setted using
`OC_USER::setBackend()` (this is done in inc/lib_user.php)
At start the choosen user manager is created
(e.g. OC_USER_DATABASE, OC_USER_LDAP)
and put into the global variable `$userManager`.
This is the variable to use instead of `OC_USER` class.
TODO: A better name than $userManager?
