Use morgan to log requests in the dev server

This is motivated by the recent switch to Express 4.
Without this commit, “make serve” will print the following message:

Error: Most middleware (like logger) is no longer bundled
with Express and must be installed separately.  Please see
https://github.com/senchalabs/connect#middleware.

server.js

@@ -10,7 +10,8 @@ var less = require("less");
 var app = express();
 
 if (require.main === module) {
-    app.use(express.logger());
+    app.use(require("morgan")(
+        ":date[iso] :method :url HTTP/:http-version - :status"));
 }
 
 var serveBrowserified = function(file, standaloneName) {