Grid system gutters

This pull request closes #68.  It changes how gutters apply to the grid system, specifically changing the concept of a "gutter" from internal to columns to external.  This more closely matches that of other grid systems.  Spacing internal to columns is now defined, appropriately, as "columnPadding."

Note that when the gutterWidth is set to 0, default row spacing collapses.  This will be fixed as part of #77.