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/allow.rs

@@ -21,7 +21,7 @@ header! {
     /// # Examples
     /// ```
     /// use hyper::header::{Headers, Allow};
-    /// use hyper::method::Method;
+    /// use hyper::Method;
     ///
     /// let mut headers = Headers::new();
     /// headers.set(
@@ -30,7 +30,7 @@ header! {
     /// ```
     /// ```
     /// use hyper::header::{Headers, Allow};
-    /// use hyper::method::Method;
+    /// use hyper::Method;
     ///
     /// let mut headers = Headers::new();
     /// headers.set(