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.

src/lib.rs

@@ -62,7 +62,7 @@
 //! }
 //! ```
 
-#![doc(html_root_url = "https://docs.rs/tokio/0.1.1")]
+#![doc(html_root_url = "https://docs.rs/tokio/0.1.2")]
 #![deny(missing_docs, warnings, missing_debug_implementations)]
 
 extern crate bytes;