Blanket rename Core to Reactor

This commit uses a script to rename `Core` to `Reactor` all at once, notably:

    find . -name '*.rs' | xargs sed -i 's/\bCore\b/Reactor/g'

tests/echo.rs

@@ -10,7 +10,7 @@ use std::thread;
 use futures::Future;
 use futures::stream::Stream;
 use tokio::net::TcpListener;
-use tokio::reactor::Core;
+use tokio::reactor::Reactor;
 use tokio_io::AsyncRead;
 use tokio_io::io::copy;
 
@@ -25,7 +25,7 @@ macro_rules! t {
 fn echo_server() {
     drop(env_logger::init());
 
-    let mut l = t!(Core::new());
+    let mut l = t!(Reactor::new());
     let srv = t!(TcpListener::bind(&t!("127.0.0.1:0".parse()), &l.handle()));
     let addr = t!(srv.local_addr());