Convert README examples to ES6

[notduncansmith]

I feel like the initial review would be a lot clearer with some changes, e.g. arrow functions.

[simplygreatwork]

Duncan, thank you so much for offering feedback. One of the nice aspects of arrow functions in ES6 are that they automatically bind to this.

Aside: I'm currently focused on adding LevelDB storage to godsend-extras. The modular stream processors in Godsend makes it pretty simple to add support for match, sort, limit, pluck, and so forth when retrieving data. It's also easy to extend. I'm hoping to publish this in the next few days.