Add flexbox-based equal-height grid columns as an experimental example

BS3 commits: 558e112de46213b352415f68eb3e05460e53be1b
grunt dist

BS3 commits: de86b36b508096eea1f9326983ab4d50358f66a5
JS: remove unused variables.

BS3 commits: ee2df11e93c88dff946560d81fd5e2d8fbdd61d4