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/entry.rs

@@ -1,6 +1,6 @@
 use Error;
 use atomic::AtomicU64;
-use timer::{Handle, Inner};
+use timer::{HandlePriv, Inner};
 
 use futures::Poll;
 use futures::task::AtomicTask;
@@ -121,7 +121,7 @@ const SHUTDOWN: *mut Entry = 1 as *mut _;
 // ===== impl Entry =====
 
 impl Entry {
-    pub fn new(when: u64, handle: Handle) -> Entry {
+    pub fn new(when: u64, handle: HandlePriv) -> Entry {
         assert!(when > 0 && when < u64::MAX);
 
         Entry {
@@ -137,7 +137,7 @@ impl Entry {
         }
     }
 
-    pub fn new_elapsed(handle: Handle) -> Entry {
+    pub fn new_elapsed(handle: HandlePriv) -> Entry {
         Entry {
             inner: handle.into_inner(),
             task: AtomicTask::new(),