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/tinydb.rs

@@ -54,7 +54,6 @@ use futures::prelude::*;
 use futures::future::Executor;
 use futures_cpupool::CpuPool;
 use tokio::net::TcpListener;
-use tokio::reactor::Handle;
 use tokio_io::AsyncRead;
 use tokio_io::io::{lines, write_all};
 
@@ -84,8 +83,7 @@ fn main() {
     // and set up our TCP listener to accept connections.
     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).expect("failed to bind");
+    let listener = TcpListener::bind(&addr).expect("failed to bind");
     println!("Listening on: {}", addr);
 
     // Create a CpuPool to execute tasks