added mobile-first functionality and offset classes

Adjusted the CSS structure to make it mobile-first; also added offset
column classes, which allow you to make layouts with whitespace without
adding in blank div columns or whatever.
housekeeping

Updated to work better with the updated grid.css
added offset and mobile grid demos

Added some rough demos to show off the new offset columns and the
mobile-only grid classes.
my math was wrong

Somehow I had messed up the calculations for columns. How embarrassing!