Fix to work with new iron api

This uses the request extensions to store the start time in the request. I played around w/ implementing it as an `AroundMiddleware`, but it seems you'd always want this to be the first before and the last after, hence it's a pair of middlewares.
Hmm. I see your point, but I think it's best to have a single type so that if you (for some reason) need to write out the type returned by Logger::new it will be easier.