Added next slide number as a second parameter to the start() callback.

This allows other JavaScript to react to the content of the next slide before the animation has been performed (no matter how the user navigates around).

By making it a second parameter this should not affect any other existing code.

Compiled the JS with CoffeeScript and minified it with UglifyJS (had to fix an indentation issue on line 519 of the `jquery.slides.coffee` file for CoffeeScript to compile it, but that was only incidental to this change).