Added a new server example that shows the use of Creationix's "Stack"…

… library as the handler.

examples/stackServer.js

@@ -0,0 +1,37 @@
+require('modbus-stack');
+require('modbus-stack/server').createServer(
+  require('stack')(
+    // Handle "Read Coils" (well, we just call `next()`...)
+    functionCode(1, function(req, res, next) {
+      console.log("Got request for '1', but passing it on...");
+      next();
+    }),
+    // Handle "Read Input Registers"
+    functionCode(4, function(req, res, next) {
+      var resp = new Array(req.quantity);
+      for (var i=0; i < req.quantity; i++) {
+        resp[i] = req.startAddress + i;
+      }
+      res.writeResponse(resp);
+    }),
+    // If all else fails, call our "Illegal Function" response.
+    illegalFunction
+  )
+).listen(502);
+
+
+function functionCode(fc, callback) {
+  return function(req, res, next) {
+    if (req.functionCode === fc) {
+      callback.apply(this, arguments);
+    } else {
+      next();
+    }
+  }
+}
+
+function illegalFunction(request, response, err) {
+  var errCode = err && err.errno ? err.errno : 1;
+  console.log("responding with exception: " + errCode);
+  response.writeException(errCode);
+}