Add support for split button dropdowns in justified button groups (with IE8 support)

The fixed values 26px/36px are calculated based on caret size, the only variable available for this is `@caret-width-base`/`@caret-width-large`.

caret size = [`.dropdown-toggle padding`](https://github.com/twbs/bootstrap/blob/master/less/button-groups.less#L107-L115) * 2 + [`.btn border`](https://github.com/twbs/bootstrap/blob/master/less/buttons.less#L18) * 2 + ( [`.caret border`](https://github.com/twbs/bootstrap/blob/master/less/dropdowns.less#L14-L15) or [`.caret border lg`](https://github.com/twbs/bootstrap/blob/master/less/button-groups.less#L135)) * 2

Sample: http://jsfiddle.net/seLk2khL