Adds setActiveLink method.

Hi, i've added a method to add a class to the currently active link. It defaults to active but it's also available in the options object as activeClass.

I've also added a menuSelector option to identify the menu html element so it's easier to target and avoid complex dom queries. This defaults to '.skrollr-menu'.

```
skrollr.menu.init(s, {
    activeClass: 'active',
    menuSelector: '.skrollr-menu'
});
```