center = function() {
            container.css({
                top: ($(window).height() - container.outerHeight()) / 2,
                left: ($(window).width() - container.outerWidth()) / 2
            });
        }
adds a button to an email attachment.

       'attachment'-parameter must be the object returned from api.dom.email().attachments().
       'contentHtml' should represent a 21x21 image of some kind. optional.
       'customCssClass' styling used on the buttons central area. optional.
       'tooltip' will be shown on hover.

       return-value is jQuery-instance representing the created button.