Jetpack: Add responsive-videos to theme-tools

Adding jetpack-responsive-videos to theme-tools to allow videos to be
responsive when theme declares support.
Merge branch 'master' of https://github.com/thomasguillot/jetpack into thomasguillot-master
Passes Grunt build process
Merge branch 'responsive-videos'
JSON API: Translate the error messages. props @georgestephanis