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/timer/now.rs

@@ -0,0 +1,27 @@
+use std::time::Instant;
+
+/// Returns `Instant` values representing the current instant in time.
+///
+/// This allows customizing the source of time which is especially useful for
+/// testing.
+pub trait Now {
+    /// Returns an instant corresponding to "now".
+    fn now(&mut self) -> Instant;
+}
+
+/// Returns the instant corresponding to now using a monotonic clock.
+#[derive(Debug)]
+pub struct SystemNow(());
+
+impl SystemNow {
+    /// Create a new `SystemNow`.
+    pub fn new() -> SystemNow {
+        SystemNow(())
+    }
+}
+
+impl Now for SystemNow {
+    fn now(&mut self) -> Instant {
+        Instant::now()
+    }
+}