Update Service trait from tokio-service to tower

[seanmonstar]

[kamyuentse]

Hi, I want to have a try, any mentoring instructions for this change?

[seanmonstar]

This wouldn't be too much work in hyper directly, but it does require tower to publish a version to crates.io. I'd probably expect it to do so only after futures 0.2, since it needs to update to the new futures.