use @screen-small var in grid.less

Per comment by @nextgenthemes on/in cd06c5baac64a366
Merge pull request #7782 from cvrebert/patch-1

use @screen-small var in grid.less
Fixes #7783: nuke bottom margin on fixed-bottom-navbar
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
correct right nav in navbar examples
Merge remote-tracking branch 'upstream/3.0.0-wip' into 3.0.0-wip