add script for npm install from github

Author: raul-casallas

package.json

@@ -17,6 +17,7 @@
   },
   "homepage": "https://github.com/modzy/sdk-javascript#readme",
   "scripts": {
+    "prepare": "babel src --out-dir=dist",
     "docs": "jsdoc -c jsdoc.json",
     "build": "babel src --out-dir=dist",
     "test": "jest --detectOpenHandles"