Fixed notice "The expiration time is greater than the session expiration" with memcache handler [Closes nette/nette#1395]

   * in case when "memcache" is set as session.save_handler and session.gc_maxlifetime is set to "0" (never expires, see http://www.php.net/manual/en/memcache.set.php)
Response: added $warnOnBuffer
Session: added getFlashSection()