Provide optional features on tokio crate (#808)

Disabling all features means the only dependency is `futures`.

Relevant pieces of the API can then be enabled with the following features:

- `codec`
- `fs`
- `io`
- `reactor`
- `tcp`
- `timer`
- `udp`
- `uds`

This also introduces the beginnings of enabling only certain pieces of the `Runtime`. As a start, the entire default runtime API is enabled via the `rt-full` feature.

src/runtime/threadpool/shutdown.rs

@@ -0,0 +1,36 @@
+use super::Inner;
+use tokio_threadpool as threadpool;
+
+use std::fmt;
+
+use futures::{Future, Poll};
+
+/// A future that resolves when the Tokio `Runtime` is shut down.
+pub struct Shutdown {
+    pub(super) inner: threadpool::Shutdown,
+}
+
+impl Shutdown {
+    pub(super) fn shutdown_now(inner: Inner) -> Self {
+        let inner = inner.pool.shutdown_now();
+        Shutdown { inner }
+    }
+}
+
+impl Future for Shutdown {
+    type Item = ();
+    type Error = ();
+
+    fn poll(&mut self) -> Poll<(), ()> {
+        try_ready!(self.inner.poll());
+        Ok(().into())
+    }
+}
+
+impl fmt::Debug for Shutdown {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Shutdown")
+            .field("inner", &"Box<Future<Item = (), Error = ()>>")
+            .finish()
+    }
+}