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/line-frames.rs

@@ -13,7 +13,7 @@ use futures::{Future, Stream, Sink};
 use futures::future::Executor;
 use futures_cpupool::CpuPool;
 use tokio::net::{TcpListener, TcpStream};
-use tokio::reactor::Core;
+use tokio::reactor::Reactor;
 use tokio_io::codec::{Encoder, Decoder};
 use tokio_io::io::{write_all, read};
 use tokio_io::AsyncRead;
@@ -55,7 +55,7 @@ impl Encoder for LineCodec {
 fn echo() {
     drop(env_logger::init());
 
-    let mut core = Core::new().unwrap();
+    let mut core = Reactor::new().unwrap();
     let handle = core.handle();
 
     let pool = CpuPool::new(1);