Undefined module for AMD loaders since 2.x

Using FileSaver.js prior to the 2.x release worked fine with AMD based loaders. Now the factory method does not return `saveAs`, so the AMD module is undefined.

Not sure how this would impact other loaders, but this would fix it for AMD users:

```
(...)
_global.saveAs = saveAs.saveAs = saveAs;

if (typeof module !== 'undefined') {
module.exports = saveAs;
}
return saveAs; // This would fix the problem with AMD based loaders
```

Thanks
Chris