Update breakpoints to em/rem

[seanwash]

http://blog.cloudfour.com/the-ems-have-it-proportional-media-queries-ftw/

I know this is an old article but it explains why I think it's a good idea. In the case of rems in media queries, ems would work the same way - when they're using in an MQ they calc their value off of the the user agent (default) font size, or the font-size set on html.