update gettting the keycodes for left and right

Fix for @fperez talk who probably might need to change his custom js to 
```js
$([IPython.events]).on('app_initialized.NotebookApp', function(){
    IPython.load_extensions('IPython-notebook-extensions/slidemode/main')
    ....
```
where `IPython-notebook-extensions/slidemode/main` is the path (without extension) to main.js if located in nbextension.

THis should make slideshow mode work with 1.x and 2.x