IE8 Button Bindings Fix and Updated README with refreshBubblePosition

--- 1 ---
 Hopscotch doesn't work in IE8. Hopscotch 0.2.0 broke it as in `_handleBubbleClick` `evt` argument will be undefined as it's not passed by IE8. Instead you have to grab the event from `window.event` object and find `currentTarget` as `evt.srcElement`.

It probably requires some testing but seems to work for me.

--- 2 ---

Updated README.md with Hopscotch's `refreshBubblePosition` API method in 0.2.0