Grunt rework

Bring the entire Grunt implementation inline with WP SEO.

Grunt tasks should be the same as before, with the addition of a `glotpress_download` task specifically for @tacoverdo :)

Also added (in the last commit) the full impact of what this all does... 

@tacoverdo @omarreiss please review :)
Would it be an idea to add `checktextdomain` here?