Adds setActiveLink method.

[pedrocorreia]

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.

[Prinzhorn]

I don't think a menu selector is needed. There can be links all over the website, not just inside a menu.

[Prinzhorn]

I added some comments. But basically I kind of doubt the usefulness. Simply adding/removing a class when the link is clicked can easily be done outside of skrollr. The tricky part is updating the state as you scroll, see #6