Modify all serve_* modules to return a Promise (preparation for #140)

src/serve_font.js

@@ -15,16 +15,19 @@ module.exports = function(options, allowedFonts) {
   var fontPath = options.paths.fonts;
 
   var existingFonts = {};
-  fs.readdir(options.paths.fonts, function(err, files) {
-    files.forEach(function(file) {
-      fs.stat(path.join(fontPath, file), function(err, stats) {
-        if (!err) {
-          if (stats.isDirectory() &&
-              fs.existsSync(path.join(fontPath, file, '0-255.pbf'))) {
-            existingFonts[path.basename(file)] = true;
+  var fontListingPromise = new Promise(function(resolve, reject) {
+    fs.readdir(options.paths.fonts, function(err, files) {
+      files.forEach(function(file) {
+        fs.stat(path.join(fontPath, file), function(err, stats) {
+          if (!err) {
+            if (stats.isDirectory() &&
+                fs.existsSync(path.join(fontPath, file, '0-255.pbf'))) {
+              existingFonts[path.basename(file)] = true;
+            }
           }
-        }
+        });
       });
+      resolve();
     });
   });
 
@@ -55,5 +58,9 @@ module.exports = function(options, allowedFonts) {
     );
   });
 
-  return app;
+  return new Promise(function(resolve, reject) {
+    fontListingPromise.then(function() {
+      resolve(app);
+    });
+  });
 };