This also moves session_start in lib/base.php down a bit because we need
OC::$SERVERROOT to get the config settings.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

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.

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?

  * 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.