Keycodes

Great little library!

Might I suggest adding a keycodes object with keycode constants?

Something like:

```
parallax.keycodes = 
{
    LEFT:37,
    TOP:38,
    RIGHT:39,
    DOWN:40
};

if(e.keyCode == p.keycodes.LEFT){  }
```

Mind you, saying that, it might just be more intuitive to add an initialization option for keyboard navigation and have keyboard navigation initialize automatically.
