Rename Sleep to Delay (#270)

This patch renames `Sleep` from tokio-timer and the tokio facade to
`Delay`. Given that the future does not actually put anything to sleep,
the `Delay` name feels more appropriate.

Fixes #263

tokio-timer/src/timer/registration.rs

@@ -8,7 +8,7 @@ use std::time::Instant;
 
 /// Registration with a timer.
 ///
-/// The association between a `Sleep` instance and a timer is done lazily in
+/// The association between a `Delay` instance and a timer is done lazily in
 /// `poll`
 #[derive(Debug)]
 pub(crate) struct Registration {