I/O resources lazily bind to reactor. (#160)

This patch makes a significant change to how I/O resources bind to a
reactor. Currently, an I/O resource (TCP, UDP, PollEvented) will bind
itself with a reactor upon creation.

First, some history.

Originally, tokio-core required that I/O resources be explicitly
associated with a reactor upon creation by passing in a `&Handle`. Tokio
reform introduced a default reactor. If I/O resources do not specify a
reactor upon creation, then the default reactor is used.

However, futures tend to favor being lazy. Creating a future should do
no work, instead it is defining a computation to be performed once the
future is executed. Binding an I/O resource with a reactor on creation
goes against this pattern.

This patch fixes this by allowing I/O resources to lazily bind to a
reactor. An explicit `&Handle` can still be used on creation, but if no
reactor is specified, then the default reactor is used. However, this
binding happens during execution time (read / write) and not creation.

src/lib.rs

@@ -107,3 +107,5 @@ pub mod runtime;
 
 pub use executor::spawn;
 pub use runtime::run;
+
+mod atomic_task;