Adds setActiveLink method.

Hi, again! 
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'
});
```

The previous problem was because i use indentation with spaces and your files seem to be indented with tabs. I've set my changes to your standard.