add experimental async/await support. (#582)

This patch adds experimental async/await support to Tokio. It does this
by adding feature flags to existing libs only where necessary in order
to add nightly specific code (mostly `Unpin` implementations). It then
provides a new crate: `tokio-async-await` which is a shim layer on top
of `tokio`.

The `tokio-async-await` crate is expected to look exactly like `tokio`
does, but with async / await support. This strategy reduces the amount
of cfg guarding in the main libraries.

This patch also adds `tokio-channel`, which is copied from futures-rs
0.1 and adds the necessary `Unpin` implementations. In general, futures
0.1 is mostly unmaintained, so it will make sense for Tokio to take over
maintainership of key components regardless of async / await support.

tokio-async-await/examples/hyper.rs

@@ -0,0 +1,25 @@
+#![feature(await_macro, async_await, futures_api)]
+
+#[macro_use]
+extern crate tokio;
+extern crate hyper;
+
+use tokio::prelude::*;
+use hyper::Client;
+
+use std::time::Duration;
+
+pub fn main() {
+    tokio::run_async(async {
+        let client = Client::new();
+
+        let uri = "http://httpbin.org/ip".parse().unwrap();
+
+        let response = await!({
+            client.get(uri)
+                .timeout(Duration::from_secs(10))
+        }).unwrap();
+
+        println!("Response: {}", response.status());
+    });
+}