added session_cache_limiter for better server response headers

Moer info here:
http://www.magentocommerce.com/boards%20/viewthread/23696/

and 

http://php.net/manual/en/function.session-cache-limiter.php#82174
@Nikita-S-Doroshenko thank's for the PR, but this tag won't have an effect on Fpc as you can see here https://github.com/GordonLesti/Lesti_Fpc/blob/master/app/code/community/Lesti/Fpc/Model/Fpc.php#L58