Return a concrete type from server::listen (#113)

* Return a concrete type from `server::listen`.
* Change `FutureServiceExt::listen` to return `(SocketAddr, Listen)`, where `Listen` is a struct created by the `service!` macro that `impls Future<Item=(),  Error=()>` and represents server execution.
* Disable `conservative_impl_trait` as it's no longer used.
* Update `FutureServiceExt` doc comment.
* Update `SyncServiceExt` doc comment. Also annotate `server::Handle` with `#[must_use]`.
* `cargo fmt`

examples/readme_errors.rs

@@ -3,7 +3,7 @@
 // Licensed under the MIT License, <LICENSE or http://opensource.org/licenses/MIT>.
 // This file may not be copied, modified, or distributed except according to those terms.
 
-#![feature(conservative_impl_trait, plugin)]
+#![feature(plugin)]
 #![plugin(tarpc_plugins)]
 
 extern crate futures;
@@ -19,7 +19,6 @@ use std::sync::mpsc;
 use std::thread;
 use tarpc::{client, server};
 use tarpc::client::sync::ClientExt;
-use tarpc::util::FirstSocketAddr;
 
 service! {
     rpc hello(name: String) -> String | NoNameGiven;