Switch back to futures from crates.io (#113)

Doing so requires copying the `current_thread` executor from GitHub into
the repo.

examples/hello.rs

@@ -19,7 +19,6 @@ extern crate tokio_io;
 use std::env;
 use std::net::SocketAddr;
 
-use futures::future;
 use futures::prelude::*;
 use tokio::net::TcpListener;
 
@@ -41,5 +40,5 @@ fn main() {
         Ok(())
     });
 
-    future::blocking(server).wait().unwrap();
+    server.wait().unwrap();
 }