Add start script for watching

Author: rattrayalex

package.json

@@ -10,6 +10,7 @@
  "test": "mocha test",
  "test:debug": "node --inspect --debug-brk mocha test",
  "build": "babel src --out-dir .",
+ "start": "babel src --out-dir . --watch",
  "preversion": "npm run build && npm test"
  },
  "author": "Alex Rattray <rattray.alex@gmail.com> (http://alexrattray.com/)",