add support for split button dropdowns in justified button groups 

Working sample: http://jsfiddle.net/0u2sa58e/

Updated sample: http://jsfiddle.net/0u2sa58e/1/

These three rules don't follow our formatting guidelines (property order and spacing). See https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md#code-guidelines and http://codeguide.co.