Fix docs to use event model for page changes

Updated the code and doc to use event instead of callback onPageChange
First commit, be gentle :-)

I moved this commit from https://github.com/openseadragon/openseadragon.github.com/pull/9
For bonus points, this "addHandler" reference could be a link to http://openseadragon.github.io/docs/OpenSeadragon.EventSource.html#addHandler.