update_session API

Currently it is required to manually perform an UPDATE query to update session data. This is a pain. For developers and MOD authors it would be nice to have an API for updating the session. I would suggest making it so:

$session->update_session($session_data, $session_id = null)

The optional session_id allows updating an other session, this however defaults to the current user's session.

Example usage (note that user extends session):

$user->update_session(array('session_page' => '/custom_page'));