Grid variations and print stylesheets

- added variations for grid (center / right aligned)
- using "only all" instead of "only screen" in media queries to have ready made print stylesheets
- moved default configuration variables back into grid.scss

Signed-off-by: Jérôme Coupé <jercoupe@gmail.com>
simplification of prefixes

Signed-off-by: Jérôme Coupé <jercoupe@gmail.com>