Add missing module descriptions

src/data_replicator.rs

@@ -1,3 +1,5 @@
+//! The `data_replicator` module implements the replication threads.
+
 use crdt;
 use packet;
 use result::Result;

src/logger.rs

@@ -1,3 +1,6 @@
+//! The `logger` module provides a setup function for `env_logger`. Its only function,
+//! `setup()` may be called multiple times.
+
 use std::sync::{Once, ONCE_INIT};
 extern crate env_logger;
 

src/timing.rs

@@ -1,3 +1,4 @@
+//! The `timing` module provides std::time utility functions.
 use std::time::Duration;
 use std::time::{SystemTime, UNIX_EPOCH};