Add no nested option

Adds an option called createNestedKeys which defaults to true.  If true the source json will be unflattened into a nested object, if false the key will not be changed.
In other words:
```js
function reverse(json) {
  return Object.keys(json).reverse().reduce(function (newObject, value) {
    newObject[value] = json[value];
    return newObject;
  }, {});
}
```