Remove grids-r.css

We'll be using [rework-pure-grids][1] to generate responsive
grids on a per-app basis. This is more useful than having
.pure-g-r inside Pure's `src/` since media queries cannot
be over-written.

[1]: https://github.com/ericf/rework-pure-grids
add .img-responsive class to base.css
remove .pure-g-r from forms manual test
add grunt-pure-grids dependency
remove grid_units grunt task, add pure_grids task
Revert "remove grid_units grunt task, add pure_grids task"

This reverts commit e91b2bef4c704134b9b367428bf665d5d77bfab7.
Revert "add grunt-pure-grids dependency"

This reverts commit d1ab35aaff9d7a438b3ac1ffa3202b3e26af9087.
Merge branch 'master' into remove-responsive-grid
update pure_grids with responsive sub-task
update .img-responsive to .pure-img
add responsive grid manual test
Use grids-responsive.css on Forms manual test
Remove grids-nr.css build file, now that there's grids-responsive.css
Merge pull request #9 from ericf/new-responsive-grids

Remove grids-nr.css and use grids-responsive.css in Forms manual test
add HISTORY.md for Grids and Base changes
Merge branch 'remove-responsive-grid' of github.com:tilomitra/pure into remove-responsive-grid
update HISTORY with related issues
Merge pull request #267 from tilomitra/remove-responsive-grid

New Responsive Grids: Better Defaults, and Customizations via Tooling or Web UI