re-export tokio-current-thread::spawn (#579)

Re-export it inside the tokio::runtime::current_thread, as the original
place (tokio::executor::current_thread) is hidden from documentation and
users need some way to spawn non-Send futures.

src/runtime/current_thread/mod.rs

@@ -71,6 +71,7 @@ mod runtime;
 
 pub use self::builder::Builder;
 pub use self::runtime::{Runtime, Handle};
+pub use tokio_current_thread::spawn;
 
 use futures::Future;