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'

examples/tinydb.rs

@@ -54,7 +54,7 @@ use futures::prelude::*;
 use futures::future::Executor;
 use futures_cpupool::CpuPool;
 use tokio::net::TcpListener;
-use tokio::reactor::Core;
+use tokio::reactor::Reactor;
 use tokio_io::AsyncRead;
 use tokio_io::io::{lines, write_all};
 
@@ -80,11 +80,11 @@ enum Response {
 }
 
 fn main() {
-    // Parse the address we're going to run this server on, create a `Core`, and
+    // Parse the address we're going to run this server on, create a `Reactor`, 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 mut core = Core::new().unwrap();
+    let mut core = Reactor::new().unwrap();
     let handle = core.handle();
     let listener = TcpListener::bind(&addr, &handle).expect("failed to bind");
     println!("Listening on: {}", addr);