Add a Handle::wakeup method (#59)

This method is intended to be used to wake up the reactor from a remote thread
if necessary, forcing it to return from a blocked call of `turn` or otherwise
prevent the next call to `turn` to from blocking.

tests/wakeup.rs

@@ -0,0 +1,43 @@
+extern crate tokio;
+
+use std::thread;
+use std::time::{Duration, Instant};
+use std::sync::mpsc;
+
+use tokio::reactor::Reactor;
+
+#[test]
+fn works() {
+    let mut r = Reactor::new().unwrap();
+    r.handle().wakeup();
+    r.turn(None);
+
+    let now = Instant::now();
+    let mut n = 0;
+    while now.elapsed() < Duration::from_millis(10) {
+        n += 1;
+        r.turn(Some(Duration::from_millis(10)));
+    }
+    assert!(n < 5);
+}
+
+#[test]
+fn wakes() {
+    const N: usize = 1_000;
+
+    let mut r = Reactor::new().unwrap();
+    let handle = r.handle();
+    let (tx, rx) = mpsc::channel();
+    let t = thread::spawn(move || {
+        for _ in 0..N {
+            rx.recv().unwrap();
+            handle.wakeup();
+        }
+    });
+
+    for _ in 0..N {
+        tx.send(()).unwrap();
+        r.turn(None);
+    }
+    t.join().unwrap();
+}