Allow returning value when requiring/importing the lib

Would be nice if we could use the lib as is:

```javascript

import angAccordion from 'ang-accordion';

angular.module('myApp', [angAccordion]);

```

instead of the current:

```javascript

import 'ang-accordion';

angular.module('myApp', ['angAccordion']);

```

If you are ok with this, I'll be happy to make a PR. :+1: 

Thanks for your library.