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/uri.rs

@@ -66,7 +66,7 @@ impl FromStr for RequestUri {
 
     fn from_str(s: &str) -> Result<RequestUri, Error> {
         let bytes = s.as_bytes();
-        if bytes == [] {
+        if bytes.len() == 0 {
             Err(Error::Uri(UrlError::RelativeUrlWithoutBase))
         } else if bytes == b"*" {
             Ok(RequestUri::Star)