Used the current plugin system to define `USER_OC_LDAP` class
that can be used as backend for OC_USER

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?

PHP function names is case-insensitive so this change isn't
intrusive but improve readability of the code.