default require() is not working

Hi,

I gave that module a try and there is a little bug which I want to mention:

### require() does not work like expected

Instead of:
``` js
const generateResponsiveImages = require('responsive-images-generator').generateResponsiveImages;
```
I have to do that:

``` js
const generateResponsiveImages = require('responsive-images-generator/lib').generateResponsiveImages;
```
In order to fix that, a little update of the main entry in your `package.json` should be enough.

### Enhancement

Would be great to get all data objects back (like an image object with path and configs) in promise handling:

``` js
generateResponsiveImages(images, configs)
.then(data => handleDataObjects(data));
```

For now I only get back an array with `undefined` items in it.