Extract the reactor to a dedicated crate. (#169)

This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.

For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.

src/runtime.rs

@@ -104,8 +104,7 @@
 //! [idle]: struct.Runtime.html#method.shutdown_on_idle
 //! [`tokio::spawn`]: ../executor/fn.spawn.html
 
-use reactor::{self, Reactor, Handle};
-use reactor::background::Background;
+use reactor::{Reactor, Handle, Background};
 
 use tokio_threadpool::{self as threadpool, ThreadPool, Sender};
 use futures::Poll;
@@ -221,7 +220,7 @@ impl Runtime {
 
         let pool = threadpool::Builder::new()
             .around_worker(move |w, enter| {
-                reactor::with_default(&handle, enter, |_| {
+                ::tokio_reactor::with_default(&handle, enter, |_| {
                     w.run();
                 });
             })