This reverts commit 6a1a4880.

* test case added to avoid adding them later

Conflicts:
	lib/private/vcategories.php

Simplifies calling these functions, and makes code simpler

functions:
deleteUser
getUsers
userExists

Move documented functions for reference into OC_USER_EXAMPLE class.

keeping tracked of the logged in user is no longer done by the active backend but by oc_user directly instead

multiply backends can be active at the same time, allowing alternative authentication procedures like openid or tokens to be used next to the regular user system

Start of the refactoring. Commit is quite big because I forgot to use git right from the beginning. Sorry.

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?

  * camelCase
  * spaces here and there

  * Using camelCase for `$classType` property
  * Using `self` keyword instead of class name
  * Added spaces here and there

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