Remove Handle argument from I/O constructors (#61)

This commit removes the `Handle` argument from the following constructors

* `TcpListener::bind`
* `TcpStream::connect`
* `UdpSocket::bind`

The `Handle` argument remains on the various `*_std` constructors as they're
more low-level, but this otherwise is intended to set forth a precedent of by
default not taking `Handle` arguments and instead relying on the global
`Handle::default` return value when necesary.

examples/hello.rs

@@ -21,15 +21,13 @@ use std::net::SocketAddr;
 
 use futures::prelude::*;
 use tokio::net::TcpListener;
-use tokio::reactor::Handle;
 
 fn main() {
     env_logger::init().unwrap();
     let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
     let addr = addr.parse::<SocketAddr>().unwrap();
 
-    let handle = Handle::default();
-    let listener = TcpListener::bind(&addr, &handle).unwrap();
+    let listener = TcpListener::bind(&addr).unwrap();
 
     let addr = listener.local_addr().unwrap();
     println!("Listening for connections on {}", addr);