use Grunt to correct the glyphicon replace format

The CSS post_process step is too late to fix this, changing the
upstream code is bad practice, the current CSS overrule leaves a
broken font link in the CSS.

None of the solutions are particularly good, but since we're already
using grunt we might as well fix this with it too.
Merge pull request #191 from ds125v/glyphicon_replace

use Grunt to correct the glyphicon replace format