Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
Well, `content-box` is the universal browser default. So everything in the CSS has always written with that assumption. These lines are just a guard in case of authors/frameworks using the star selector and resetting the box model globally at the top of the stylesheet, which would break responsive percentage padding. Lots of folks do use the star selector fix a la Paul Irish, e.g. <a href="https://github.com/twbs/bootstrap/blob/49519e981614fb8e0c4fabcbb0a632db292c2b3c/less/scaffolding.less">Bootstrap</a>. This isn't a change so much as insurance that things stay the same.