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-threadpool/README.md

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+# Tokio Thread Pool
+
+A library for scheduling execution of futures concurrently across a pool of
+threads.
+
+**Note**: This library isn't quite ready for use.
+
+### Why not Rayon?
+
+Rayon is designed to handle parallelizing single computations by breaking them
+into smaller chunks. The scheduling for each individual chunk doesn't matter as
+long as the root computation completes in a timely fashion. In other words,
+Rayon does not provide any guarantees of fairness with regards to how each task
+gets scheduled.
+
+On the other hand, `tokio-threadpool` is a general purpose scheduler and
+attempts to schedule each task fairly. This is the ideal behavior when
+scheduling a set of unrelated tasks.
+
+### Why not futures-cpupool?
+
+It's 10x slower.
+
+## Examples
+
+```rust
+extern crate tokio_threadpool;
+extern crate futures;
+
+use tokio_threadpool::*;
+use futures::*;
+use futures::sync::oneshot;
+
+pub fn main() {
+    let (tx, _pool) = ThreadPool::new();
+
+    let res = oneshot::spawn(future::lazy(|| {
+        println!("Running on the pool");
+        Ok::<_, ()>("complete")
+    }), &tx);
+
+    println!("Result: {:?}", res.wait());
+}
+```
+
+## License
+
+`tokio-threadpool` is primarily distributed under the terms of both the MIT
+license and the Apache License (Version 2.0), with portions covered by various
+BSD-like licenses.
+
+See LICENSE-APACHE, and LICENSE-MIT for details.