Duplicate registration of listeners after calling ShareButtons.update()

[wcoder]

Reported by @tnga in a possible fix #28

Previous added listeners to share buttons are re-added twice or multiple time when execute ShareButtons.update().
In browsers, if data-url or data-title or data-desc have been changed before calling ShareButtons.update(), the changes will not be considered because the previous added listeners with previous data value on share buttons will be executed on click event before the new added listeners after ShareButtons.update().

[wcoder]

Fixed by #30