Add missed "use strict"

lib/worker.js

@@ -3,10 +3,12 @@
  * addon and makes it available for Node.
  */
 
+"use strict";
+
 try {
   module.exports = require("../build/Release/worker");
 } catch(e) {
   // Do nothing for a moment. Everything will work except the functions
-  // that uses worker routines.
+  // that use worker routines.
   // TODO(Kagami) Provide pure JS fallback.
 }