Support frameworks that add extra elements to your markup

I really like Neat but right now it's not a good fit for an Ember application.  Ember adds extra "metamorph" script tags to the markup which means that trying to strip the margin on the last-child won't work for `span-columns()`.

This pull request uses the last-of-type selector to target the last element that you would actually want to remove margin from.  Unfortunately this selector doesn't work on IE8 and earlier.  Still, I think this is a useful feature.