Implement Default for tokio_timer::Handle (#553)

This patch implements `Default` for `tokio_timer::Handle`. It returns a
`Handle` instance that is not bound to a specific timer. Instead, it
will use the timer for the current execution context. This is the same
strategy used by `tokio_reactor::Handle`.

Fixes #547

tokio-timer/src/timer/mod.rs

@@ -38,6 +38,7 @@ mod now;
 mod registration;
 
 use self::entry::Entry;
+use self::handle::HandlePriv;
 use self::level::{Level, Expiration};
 
 pub use self::handle::{Handle, with_default};