Extract the reactor to a dedicated crate. (#169)

This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.

For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.

tokio-executor/src/lib.rs

@@ -1,15 +1,34 @@
-//! Task execution utilities.
+//! Task execution related traits and utilities.
 //!
 //! In the Tokio execution model, futures are lazy. When a future is created, no
 //! work is performed. In order for the work defined by the future to happen,
 //! the future must be submitted to an executor. A future that is submitted to
 //! an executor is called a "task".
 //!
-//! The executor executor is responsible for ensuring that [`Future::poll`] is
-//! called whenever the task is [notified]. Notification happens when the
-//! internal state of a task transitions from "not ready" to ready. For
-//! example, a socket might have received data and a call to `read` will now be
-//! able to succeed.
+//! The executor is responsible for ensuring that [`Future::poll`] is called
+//! whenever the task is [notified]. Notification happens when the internal
+//! state of a task transitions from "not ready" to ready. For example, a socket
+//! might have received data and a call to `read` will now be able to succeed.
+//!
+//! This crate provides traits and utilities that are necessary for building an
+//! executor, including:
+//!
+//! * The [`Executor`] trait describes the API for spawning a future onto an
+//!   executor.
+//!
+//! * [`enter`] marks that the the current thread is entering an execution
+//!   context. This prevents a second executor from accidentally starting from
+//!   within the context of one that is already running.
+//!
+//! * [`DefaultExecutor`] spawns tasks onto the default executor for the current
+//!   context.
+//!
+//! * [`Park`] abstracts over blocking and unblocking the current thread.
+//!
+//! [`Executor`]: trait.Executor.html
+//! [`enter`]: fn.enter.html
+//! [`DefaultExecutor`]: struct.DefaultExecutor.html
+//! [`Park`]: park/index.html
 
 #![deny(missing_docs, missing_debug_implementations, warnings)]
 #![doc(html_root_url = "https://docs.rs/tokio-executor/0.1")]