Namespacing and refactoring

This PR includes substantial changes to the *how* CSS code is generated (for the sake of configurability and encapsulation), but little change to the resulting CSS. Changes include:

* Vars and mxins are new prefixed w/`cg-` to prevent collisions/confusion when using other libraries
* The default size that gets used by browsers that cannot understand media queries is now configurable via `$cg-default-size-prefix` (which accepts the size identifiers `xs`, `sm`, `md`, `lg`) 
* The code that generates grid styles for each size is now generated in a mixin called `_cg-generate-for-size()`. Aside from the `xs` size, it generates code for each using the same routine and replacing various values with the size-specific variables defined early in the script.