Swap Handle/Pinned

* Handle -> Remote
* Pinned -> Handle

All APIs now take a `&Handle` by default and in general can return an immediate
`io::Result` instead of an `IoFuture`. This reflects how most usage will likely
be done through handles rather than remotes, and also all previous functionality
can be recovered with a `oneshot` plus `Remote::spawn`.

Closes #15

tests/echo.rs

@@ -24,8 +24,7 @@ fn echo_server() {
     drop(env_logger::init());
 
     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 srv = t!(TcpListener::bind(&t!("127.0.0.1:0".parse()), &l.handle()));
     let addr = t!(srv.local_addr());
 
     let msg = "foo bar baz";