Add some missing future::Executor implementations (#563)

This adds an implementation of future::Executor for
`executor::DefaultExecutor` and `runtime::current_thread::Handle`.

tokio-executor/tests/executor.rs

@@ -2,10 +2,27 @@ extern crate tokio_executor;
 extern crate futures;
 
 use tokio_executor::*;
-use futures::future::lazy;
+use futures::{Future, future::lazy};
 
-#[test]
-fn spawn_out_of_executor_context() {
-    let res = DefaultExecutor::current().spawn(Box::new(lazy(|| Ok(()))));
-    assert!(res.is_err());
+mod out_of_executor_context {
+    use super::*;
+
+    fn test<F, E>(spawn: F)
+    where
+        F: Fn(Box<Future<Item=(), Error=()> + Send>) -> Result<(), E>,
+    {
+        let res = spawn(Box::new(lazy(|| Ok(()))));
+        assert!(res.is_err());
+    }
+
+    #[test]
+    fn spawn() {
+        test(|f| DefaultExecutor::current().spawn(f));
+    }
+
+    #[test]
+    fn execute() {
+        use futures::future::Executor as FuturesExecutor;
+        test(|f| DefaultExecutor::current().execute(f));
+    }
 }