I'm pretty sure that only Congress has the power to legislate the end of the war. Now, I suppose it's possible there could be a joint resolution. Maybe a better scholar than I could answer that. Maybe a constitutional scholar.
As for ending the war, yes, he most definitely has a plan. It's on his site. It is much like John Kerry's was in 2004. Redeploy. Pulls the troops out gradually. Concert effort into political and diplomatic solutions as opposed to armed conflict.
Barack Obama has been speaking against going to war with Iraq since 2002:
http://youtube.com/watch?v=sXzmXy226po