Runtime builder (#234)

* Split runtime module into files
* Add runtime::Builder to set up thread pool.

src/runtime/shutdown.rs

@@ -0,0 +1,46 @@
+use runtime::Inner;
+
+use std::fmt;
+
+use futures::{Future, Poll};
+
+/// A future that resolves when the Tokio `Runtime` is shut down.
+pub struct Shutdown {
+    pub(super) inner: Box<Future<Item = (), Error = ()> + Send>,
+}
+
+impl Shutdown {
+    pub(super) fn shutdown_now(inner: Inner) -> Self {
+        let inner = Box::new({
+            let pool = inner.pool;
+            let reactor = inner.reactor;
+
+            pool.shutdown_now().and_then(|_| {
+                reactor.shutdown_now()
+                    .then(|_| {
+                        Ok(())
+                    })
+            })
+        });
+
+        Shutdown { inner }
+    }
+}
+
+impl Future for Shutdown {
+    type Item = ();
+    type Error = ();
+
+    fn poll(&mut self) -> Poll<(), ()> {
+        try_ready!(self.inner.poll());
+        Ok(().into())
+    }
+}
+
+impl fmt::Debug for Shutdown {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Shutdown")
+            .field("inner", &"Box<Future<Item = (), Error = ()>>")
+            .finish()
+    }
+}