add support for split button dropdowns in justified button groups 

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

Size variation can be implemented using:
```
(@padding-base-horizontal * 2  + @font-size-base  + 2px) /* 2px from .caret*/ 
```