Added support for usercreation from Moodle to authentication module.

Added support for usercreation from Moodle to authentication module.
Works currently only with ldap-module (tested with e-directory), but
 other modules could be extended too by
adding following functions:

function auth_user_exists ($username) {
//returns true if given username  already exists on authetication database
}

function auth_user_create ($userobject,$plainpass) {
//create new user to authentication database
//in inactive state (if posible)
//returns true if user is created
}
function auth_user_activate ($username) {
//activate external user  after email-address is confirmed
//returns true if user is activated
}