add support for split button dropdowns in justified button groups 

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

For some reason I didn't think about this earlier, but this seems rather fragile and too specific. Using a fixed value won't work for anyone who customizes the `padding`, `font-size`, etc.