Added "down" media queries

Added media queries with a max-width and no min-width.

It would be nice to have media queries that catch everything less than a given size. I know the philosophy of Zurb Foundation is mobile first, but there are times when the default/existing styles are correct for desktop, but need to be adjusted only for mobile. Having the appropriate media queries will save excess code.

I know $small-down, and $xxlarge-down are both redundant, but so are $small-up, and $xxlarge-up, so I've included them here for completeness sake, perhaps they could be ommited.

Also, in the guidlines for contributing, it was instructed to update the documentation, but I didn't see where the documentation is, so I haven't done so (sorry). Perhaps that could be added to the instructions.