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

Created by: MohammadYounes

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 * 2 + .btn border * 2 + ( .caret border or .caret border lg) * 2

Sample: http://jsfiddle.net/seLk2khL