Return ISO 8601 compliant return for dates in API

The API currently returns nicely formatted dates for `created_at` and `updated_at` values but it would be more desirable to go for a format in line with [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601).

Particular use-case would be implementing [moment.js](http://momentjs.com/docs/#/parsing/).

Here is a good reference - https://github.com/moment/moment/issues/1407
