Update BusyButton.js

* Workaround Unknown Runtime Error
** in some versions of Internet Explorer, an Unknown Runtime Error is thrown when the innerHTML of a DOM node is altered by a function which was invoked by a member of the innerHTML
** since BusyButton#setLabel empties the containerNode, we can avoid this issue by simply appending a new TextNode to the containerNode (which should behave the same as setting the innerHTML to a String literal)