Category Layouts, MSRP

Added support for the admin to select a layout for their category view. See the Category Browse template for the default implimentation.

Added MSRP for products. Check out the _product partial for default usage. (Strike-through if it is greater than the variant's price).

Also, have added the default partials templates to this docs site. For instance, when you call {{ include 'product' with product }} it loads the _product partial with the product variable.