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/method.rs

@@ -7,8 +7,6 @@ use error::Error;
 use self::Method::{Options, Get, Post, Put, Delete, Head, Trace, Connect, Patch,
                    Extension};
 
-#[cfg(feature = "serde-serialization")]
-use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 /// The Request Method (VERB)
 ///
@@ -134,21 +132,6 @@ impl Default for Method {
     }
 }
 
-#[cfg(feature = "serde-serialization")]
-impl Serialize for Method {
-    fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error> where S: Serializer {
-        format!("{}", self).serialize(serializer)
-    }
-}
-
-#[cfg(feature = "serde-serialization")]
-impl Deserialize for Method {
-    fn deserialize<D>(deserializer: &mut D) -> Result<Method, D::Error> where D: Deserializer {
-        let string_representation: String = try!(Deserialize::deserialize(deserializer));
-        Ok(FromStr::from_str(&string_representation[..]).unwrap())
-    }
-} 
-
 #[cfg(test)]
 mod tests {
     use std::collections::HashMap;