update to es version supported by node 4.x+
update to es version supported by node 4.x+

- eliminated `self = this`, replaced `self` with `this`
- replaced callbacks with arrow functions
- replaced var with const and let, used let in the opening expression of for loops to support node 4.x

followed this guide: https://node.green/#ES2015-functions-arrow-functions
shama/gaze