test
Replace hardcoded values with dynamic width and push/pull generation

Originally, csswizardry-grids used hardcoded values to generate the
widths and offsets for its classes. This version uses a single mixin
for column widths, as well as push and pull offsets, for an arbitrary
set of fractional widths — the file provided will generate the same set
of values as the original csswizardry-grids, but can be used for any
combination of values desired.