Use a timer heap instead of a timer wheel

In general it's easier to implement and should have more predictable performance
semantics for applications in general. More serious timer usage can go through
`tokio-timer` which has properly configurable timer wheels and such.

Closes #2
Closes #7

src/lib.rs

@@ -109,7 +109,7 @@ extern crate log;
 pub mod io;
 
 mod mpsc_queue;
-mod timer_wheel;
+mod heap;
 pub mod channel;
 pub mod net;
 pub mod reactor;