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.

src/lib.rs

@@ -49,16 +49,13 @@
 //! use tokio_io::AsyncRead;
 //! use tokio_io::io::copy;
 //! use tokio::net::TcpListener;
-//! use tokio::reactor::Handle;
 //!
 //! fn main() {
-//!     let handle = Handle::default();
-//!
 //!     let pool = CpuPool::new_num_cpus();
 //!
 //!     // Bind the server's socket.
 //!     let addr = "127.0.0.1:12345".parse().unwrap();
-//!     let listener = TcpListener::bind(&addr, &handle)
+//!     let listener = TcpListener::bind(&addr)
 //!         .expect("unable to bind TCP listener");
 //!
 //!     // Pull out a stream of sockets for incoming connections