major grid changes

reapproaching grid methodology and structure. created "grid-of-$"
support. all grids are now defined in pixel widths with support for four
breakpoints (768, 1024, 1224, 1824). below 768, all columns get
width:auto as expected. further commits will reinstate .grid-forced
(percentage based for < 768, pixel-defined for higher); also need
full-width support. check __devnotes.txt for more information.