Bump tokio to v0.1.2 (#201)

This also bumps tokio-io to v0.1.6 and prepares for the initial release
of tokio-executor, tokio-reactor, and tokio-threadpool.

tokio-threadpool/src/lib.rs

@@ -1,5 +1,6 @@
 //! A work-stealing based thread pool for executing futures.
 
+#![doc(html_root_url = "https://docs.rs/tokio-threadpool/0.1.0")]
 #![deny(warnings, missing_docs, missing_debug_implementations)]
 
 extern crate tokio_executor;