Add classHeader option

An optional parameter to set a class in the header in a dialog.

Example:
       bootbox.dialog({
        classHeader: "alert-info",
        message: "I am a custom dialog",
        title: "Custom title",
        buttons: {
          success: {
            label: "Success!",
            className: "btn-success",
            callback: function() {
              Example.show("great success");
            }
          },
          danger: {
            label: "Danger!",
            className: "btn-danger",
            callback: function() {
              Example.show("uh oh, look out!");
            }
          },
          main: {
            label: "Click ME!",
            className: "btn-primary",
            callback: function() {
              Example.show("Primary button");
            }
          }
        }
      });