add index.styl for easy inclusion

nib has an [`index.styl`](https://github.com/visionmedia/nib/blob/master/index.styl) file in the home directory so that you can easily include it with `@import 'nib'` rather than, say, `@import 'nib/lib/nib/'. Made things more convenient for me in a current project where using npm isn't clean or necessary (on harp.io).