Allow voting based on session cookies

Problem/Motivation

I have a project requiring session based voting. Rather than simply setting the cookie timeout to be really short (say 15 mins - a likely length session for the site) the client is saying we need to implement it on a session cookie.

Proposed resolution

I have written a patch to do this and submit it here as my client can't be the only one who will ever need session based voting (Although it's as easy to spoof as cookie based voting is anyway)