Threadpool blocking (#317)

This patch adds a `blocking` to `tokio-threadpool`. This function serves
as a way to annotate sections of code that will perform blocking
operations. This informs the thread pool that an additional thread needs
to be spawned to replace the current thread, which will no longer be
able to process the work queue.

tokio-threadpool/src/shutdown.rs

@@ -1,8 +1,6 @@
 use pool::Pool;
 use sender::Sender;
 
-use std::sync::atomic::Ordering::{Acquire};
-
 use futures::{Future, Poll, Async};
 #[cfg(feature = "unstable-futures")]
 use futures2;
@@ -35,11 +33,10 @@ impl Future for Shutdown {
 
     fn poll(&mut self) -> Poll<(), ()> {
         use futures::task;
-        trace!("Shutdown::poll");
 
         self.inner().shutdown_task.task1.register_task(task::current());
 
-        if 0 != self.inner().num_workers.load(Acquire) {
+        if !self.inner().is_shutdown() {
             return Ok(Async::NotReady);
         }