Improving, and adding back some missing features

Hi Lucas,

I've implemented some updates to your excellent Stylus port of CSSWizardry Grids.

* Added an option to switch between `inline-block` or `float` scaffolding.
* Added back optional shadow classes. The matter with those guys is what the preprocessor does when you *don't* extend a certain class.
* Added the `inline-block` spacing fix detailed [here](github.com/csswizardry/inuit.css/issues/170#issuecomment-14859371).
* Added customizable class names. This is something the original Sass version doesn't have, yet is quite useful.

Hope this deserves a pull! Cheers! :)