rename .retina-image => .img-retina
Update forms.less

All variables should be set in variables.less?
rm .control-group from docs since it's no longer in the LESS
Merge pull request #8455 from cvrebert/control-group-dead

rm .control-group from docs since it's no longer in the LESS
Merge pull request #8454 from bassjobsen/patch-2

Update forms.less
Merge pull request #8453 from cvrebert/retina-rename

rename .retina-image => .img-retina
Update customizer

Add all the new things to customizer:
* `@headings-font-family`
* `@container-*`
* darker `@btn-default-bg`
Merge pull request #8478 from juthilo/3.0.0-customizer

Update customizer
fix broken scrollspy example
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip