feat(lib): add support to disable tokio-proto internals

For now, this adds `client::Config::no_proto`, `server::Http::no_proto`,
and `server::Server::no_proto` to skip tokio-proto implementations, and
use an internal dispatch system instead.

`Http::no_proto` is similar to `Http::bind_connection`, but returns a
`Connection` that is a `Future` to drive HTTP with the provided service.
Any errors prior to parsing a request, and after delivering a response
(but before flush the response body) will be returned from this future.

See #1342 for more.

.travis.yml

@@ -8,6 +8,8 @@ matrix:
           env: FEATURES="--features nightly"
         - rust: beta
         - rust: stable
+        - rust: stable
+          env: HYPER_NO_PROTO=1
         - rust: stable
           env: FEATURES="--features compat"
         - rust: 1.17.0