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
Let's combine these into a single call:

```
viewer.addHandler("page", function (data) {
    document.getElementById('currentpage').innerHTML = ( data.page + 1 ) + ' of 3';
});
```