Implement Default for tokio_timer::Handle (#553)

This patch implements `Default` for `tokio_timer::Handle`. It returns a
`Handle` instance that is not bound to a specific timer. Instead, it
will use the timer for the current execution context. This is the same
strategy used by `tokio_reactor::Handle`.

Fixes #547

tokio-timer/tests/delay.rs

@@ -7,6 +7,7 @@ mod support;
 use support::*;
 
 use tokio_timer::*;
+use tokio_timer::timer::Handle;
 
 use futures::Future;
 
@@ -486,3 +487,19 @@ fn reset_future_delay_after_fire() {
         assert_ready!(delay);
     });
 }
+
+#[test]
+fn delay_with_default_handle() {
+    let handle = Handle::default();
+    let now = Instant::now();
+
+    let mut delay = handle.delay(now + ms(1));
+
+    mocked_with_now(now, |timer, _time| {
+        assert_not_ready!(delay);
+
+        turn(timer, ms(1));
+
+        assert_ready!(delay);
+    });
+}