Reorganize the entire crate:

Renamed APIs

* Loop => reactor::Core
* LoopHandle => reactor::Handle
* LoopPin => reactor::Pinned
* TcpStream => net::TcpStream
* TcpListener => net::TcpListener
* UdpSocket => net::UdpSocket
* Sender => channel::Sender
* Receiver => channel::Receiver
* Timeout => reactor::Timeout
* ReadinessStream => reactor::PollEvented
* All `LoopHandle` methods to construct objects are now free functions on the
  associated types, e.g. `LoopHandle::tcp_listen` is now `TcpListener::bind`
* All APIs taking a `Handle` now take a `Handle` as the last argument
* All future-returning APIs now return concrete types instead of trait objects

Added APIs

* io::Io trait -- Read + Write + ability to poll

Removed without replacement:

* AddSource
* AddTimeout
* IoToken
* TimeoutToken

Closes #3
Closes #6

tests/echo.rs

@@ -9,6 +9,8 @@ use std::thread;
 use futures::Future;
 use futures::stream::Stream;
 use tokio_core::io::{copy, TaskIo};
+use tokio_core::net::TcpListener;
+use tokio_core::reactor::Core;
 
 macro_rules! t {
     ($e:expr) => (match $e {
@@ -21,8 +23,8 @@ macro_rules! t {
 fn echo_server() {
     drop(env_logger::init());
 
-    let mut l = t!(tokio_core::Loop::new());
-    let srv = l.handle().tcp_listen(&"127.0.0.1:0".parse().unwrap());
+    let mut l = t!(Core::new());
+    let srv = TcpListener::bind(&t!("127.0.0.1:0".parse()), &l.handle());
     let srv = t!(l.run(srv));
     let addr = t!(srv.local_addr());