Switch back to futures from crates.io (#113)

Doing so requires copying the `current_thread` executor from GitHub into
the repo.

examples/tinyhttp.rs

@@ -90,7 +90,7 @@ fn worker(rx: mpsc::UnboundedReceiver<net::TcpStream>) {
         })).unwrap();
         Ok(())
     });
-    future::blocking(done).wait().unwrap();
+    done.wait().unwrap();
 }
 
 /// "Server logic" is implemented in this function.