reduce the number of files in the npm package

Reduce the number of files in the npm package by using the files array
whitelist. README.md and LICENCE are automatically included by npm:

https://docs.npmjs.com/files/package.json#files

Author: silverwind

.npmignore

@@ -28,5 +28,8 @@
   "devDependencies": {
     "bl": "~0.9.3",
     "yauzl": "~2.3.1"
-  }
+  },
+  "files": [
+    "index.js"
+  ]
 }