Extract the reactor to a dedicated crate. (#169)

This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.

For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.

README.md

@@ -87,7 +87,30 @@ fn main() {
 }
 ```
 
-# License
+## Project layout
+
+The `tokio` crate, found at the root, is primarily intended for use by
+application developers.  Library authors should depend on the sub crates, which
+have greater guarantees of stability.
+
+The crates included as part of Tokio are:
+
+* [`tokio-executor`]: Task execution related traits and utilities.
+
+* [`tokio-io`]: Asynchronous I/O related traits and utilities.
+
+* [`tokio-reactor`]: Event loop that drives I/O resources (like TCP and UDP
+  sockets).
+
+* [`tokio-threadpool`]: Schedules the execution of futures across a pool of
+  threads.
+
+[`tokio-executor`]: tokio-executor
+[`tokio-io`]: tokio-io
+[`tokio-reactor`]: tokio-reactor
+[`tokio-threadpool`]: tokio-threadpool
+
+## License
 
 This project is licensed under either of