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/lib.rs

@@ -74,6 +74,7 @@ extern crate slab;
 #[macro_use]
 extern crate tokio_io;
 extern crate tokio_executor;
+extern crate tokio_reactor;
 extern crate tokio_threadpool;
 
 #[macro_use]
@@ -87,8 +88,6 @@ pub mod runtime;
 pub use executor::spawn;
 pub use runtime::run;
 
-mod atomic_task;
-
 pub mod io {
     //! Asynchronous I/O.
     //!