[chore] Restrict files included in npm package (#57)

Author: darrachequesne

package.json

@@ -7,6 +7,9 @@
     "type": "git",
     "url": "https://github.com/socketio/socket.io-emitter.git"
   },
+  "files": [
+    "index.js"
+  ],
   "scripts": {
     "test": "make test"
   },