Adding Kerning

Thanks for the awesome work so far. I need a couple extra features (kerning and shape vectors) so I'll try to add them in the next week or so. I'll try to keep it consistent with FreeType API. I'm still new to C/nan so let me know if there's any better way of doing this.

Added these methods and some code in the example:
```
ft.HAS_KERNING( face )
ft.Get_Kerning( face, leftIndex, rightIndex, kernMode, kernObject )
```

`Get_Kerning` will return 0 for success, and puts 'x' and 'y' into the object.

Will add some tests in the next few days. 


I also aded `npm run build` to the scripts.