Added posibility to set style (CSS class) of TabBarButton's badge

Currently it is very inconvinient to change class of TabBarButton's badge. Eg.

    // ensure tab_button.badgeObj is created
    tab_button._setBadgeAttr(null);
    // remove default red style
    domClass.remove(tab_button.badgeObj.domNode,
                    tab_button.badgeObj.className);
    tab_button.badgeObj.className = 'mblDomButtonGrayBadge';
    // add new gray style
    domClass.add(tab_button.badgeObj.domNode,
                 tab_button.badgeObj.className);