Add *http.Request to Session struct allowing you to inspect the initial ajax request

This exists on the master branch with https://github.com/googollee/go-socket.io/blob/master/socket.go#L19, but I couldn't find a similar way to access this information on the 0.9.x branch

This was a quick hack, so please review critically and I would love to fix it up or put this information somewhere else if that is better.

thanks!