feat(lib): redesign API to use Futures and Tokio

There are many changes involved with this, but let's just talk about
user-facing changes.

- Creating a `Client` and `Server` now needs a Tokio `Core` event loop
to attach to.
- `Request` and `Response` both no longer implement the
`std::io::{Read,Write}` traits, but instead represent their bodies as a
`futures::Stream` of items, where each item is a `Chunk`.
- The `Client.request` method now takes a `Request`, instead of being
used as a builder, and returns a `Future` that resolves to `Response`.
- The `Handler` trait for servers is no more, and instead the Tokio
`Service` trait is used. This allows interoperability with generic
middleware.

BREAKING CHANGE: A big sweeping set of breaking changes.

src/header/common/content_length.rs

@@ -72,7 +72,6 @@ impl fmt::Display for ContentLength {
 }
 
 __hyper__deref!(ContentLength => u64);
-__hyper_generate_header_serialization!(ContentLength);
 
 __hyper__tm!(ContentLength, tests {
     // Testcase from RFC