Support the Rack hijack API

Rack now supports a "hijack" API which allows high-level frameworks built on top of Rack (e.g. Rails) to access the underlying socket:

https://github.com/rack/rack/pull/481

It would be great if Reel supported this feature, as it would expose its end-to-end streaming support (along with Celluloid::IO-based sockets) to frameworks wishing to utilize the hijack API.