Introduce the Tokio runtime: Reactor + Threadpool (#141)

This patch is an intial implementation of the Tokio runtime. The Tokio
runtime provides an out of the box configuration for running I/O heavy
asynchronous applications.

As of now, the Tokio runtime is a combination of a work-stealing thread
pool as well as a background reactor to drive I/O resources.

This patch also includes tokio-executor, a hopefully short lived crate
that is based on the futures 0.2 executor RFC.

* Implement `Park` for `Reactor`

This enables the reactor to be used as the thread parker for executors.
This also adds an `Error` component to `Park`. With this change, a
`Reactor` and a `CurrentThread` can be combined to achieve the
capabilities of tokio-core.

tokio-executor/tests/executor.rs

@@ -0,0 +1,11 @@
+extern crate tokio_executor;
+extern crate futures;
+
+use tokio_executor::*;
+use futures::future::lazy;
+
+#[test]
+fn spawn_out_of_executor_context() {
+    let res = DefaultExecutor::current().spawn(Box::new(lazy(|| Ok(()))));
+    assert!(res.is_err());
+}