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.
The reason I didn't `impl BeforeMiddleware for Logger` is that it seemed confusing to create a `Logger` that wouldn't use its member `Option<Format>`. But if this is your preference I'll make the change.