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.

.travis.yml

@@ -2,15 +2,10 @@ language: rust
 matrix:
     fast_finish: true
     include:
-        - os: osx
-          rust: stable
-          env: FEATURES="--no-default-features --features security-framework"
         - rust: nightly
           env: FEATURES="--features nightly"
         - rust: beta
         - rust: stable
-        - rust: stable
-          env: FEATURES="--no-default-features"
 
 cache:
     apt: true