added math.md

Added math.md, a simple Markdown doc which notes how to work out your
grid measurements – helps with measuring a customised grid with
different column numbers and margin sizes.
Removed css3-mediaqueries.js

Not needed right now (might replace with something else later).
Updated grid demo

Altered the existing grid demo to show off the column sizes better.

Also added in a demo for some offset columns.
commented

Added a quick comment that the default styles can be cleared out.
updated class rules

Updated the CSS class rules to match the newer syntax in grid.css

Not sure why this is needed, but I’ll keep it until I can do some tests.
simplified media queries

Simplified the mobile media queries. Should make it easier to adapt the
code for mobile-first later on.
god i hate ds store