Added updateSessionOptions function to res for updating session options.

Added a function to the response object which allows updating of session options. Use case for me was a "remember me" login form. In other words, this function resolves #15.

Example Usage: `res.updateSessionOptions({expires : expiry});` where `expiry` is a javascript Date object.

If this breaks anything, please just reject this pull request. I haven't throughly tested this outside my own use case. Just submitting a pull request in case it might be useful.