Provide a timer implementation (#249)

This patch adds a new crate: tokio-timer. This crate provides an
efficient timer implemeentation designed for use in Tokio based
applications.

The timer users a hierarchical hashed timer wheel algorithm with six
levels, each having 64 slots. This allows the timer to have a resolution
of 1ms while maintaining O(1) complexity for insert, removal, and firing
of timeouts.

There already exists a tokio-timer crate. This is a complete rewrite
which solves the outstanding problems with the existing tokio-timer
library.

Closes #146.

tokio-timer/src/lib.rs

@@ -0,0 +1,41 @@
+//! Utilities for scheduling work to happen after a period of time.
+//!
+//! This crate provides a number of utilities for working with periods of time:
+//!
+//! * [`Sleep`]: A future that completes at a specified instant in time.
+//!
+//! * [`Interval`] A stream that yields at fixed time intervals.
+//!
+//! * [`Deadline`]: Wraps a future, requiring it to complete before a specified
+//!   instant in time, erroring if the future takes too long.
+//!
+//! These three types are backed by a [`Timer`] instance. In order for
+//! [`Sleep`], [`Interval`], and [`Deadline`] to function, the associated
+//! [`Timer`] instance must be running on some thread.
+//!
+//! [`Sleep`]: struct.Sleep.html
+//! [`Deadline`]: struct.Deadline.html
+//! [`Interval`]: struct.Interval.html
+//! [`Timer`]: timer/struct.Timer.html
+
+#![doc(html_root_url = "https://docs.rs/tokio-timer/0.2.0")]
+#![deny(missing_docs, warnings, missing_debug_implementations)]
+
+extern crate tokio_executor;
+
+#[macro_use]
+extern crate futures;
+
+pub mod timer;
+
+mod atomic;
+mod deadline;
+mod error;
+mod interval;
+mod sleep;
+
+pub use self::deadline::{Deadline, DeadlineError};
+pub use self::error::Error;
+pub use self::interval::Interval;
+pub use self::timer::{Timer, with_default};
+pub use self::sleep::Sleep;