Added compress demo images script

Author: manuel12

package.json

@@ -6,7 +6,8 @@
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
-    "build": "gulp"
+    "build": "gulp",
+    "compress-demo-images": "gulp compressDemoImages"
   },
   "keywords": [],
   "author": "",