Replace Makefile with GruntJS

A rebase (against soon-to-be 3.0.0-rc.1) & squash of
https://github.com/twbs/bootstrap/pull/7786 AKA
https://github.com/twitter/bootstrap/pull/7786
originally by
@seriema
@mokkabonna
@jojohess

Rebased by @cvrebert