Removed global vars are DOCUMENTROOT, SERVERROOT, SUBURI, WEBROOT and CONFIG_DATADIRECTORY

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

patch by Debayan Banerjee

new settings page that allows plugins to integrate their own settings dialogs more with the existing ones

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?

  * Support numeric types too
  * $WEBROOT variable was not necessary

This actually write all the key/value into the config.php.
The code in `writeAdminLisener()` is cleaner.
This is in prevision of new configuration options (e.g. plugin related)

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