kickoff 1.5.0
set attribute-free shortcode grid size and class parameters

upstream default 4 doesn't make sense with new pure
gc_column_class must always return pure-u-1 for default column collapse

all media query column widths are specified with column classes in
content pages. shortcode grid and span parameters are for grouping
columns into pure-g elements.
pure-g-r doesn't exist anymore
hybrid base dynamic main content grid
since 1-col grid is default, probably have to allow it too, fixup 9754ee8

1-column "grids" are useful for creating special-styled content
blocks, such a small-font t&c section at the end of a content page etc
support new grids-responsive approach
bump pure 0.5.0
Merge branch '1.5.0'