Added way to specify custom classes to the buttons

This PR allows the developers to set their own classes to the buttons of the helper layer.

For me, it looks like this:

![screen shot 2013-05-22 at 11 34 32 am](https://f.cloud.github.com/assets/696982/548843/c157dc30-c2ec-11e2-8e2a-b74156d8f61b.png)

All you have to do is pass the class to the `buttonCls` property of IntroJs. 

If you use Twitter Bootstrap, for instance, you can pass its default button class:

    introJs.setOptions({
        buttonCls: 'btn'
    });

Or even using the `setOption` method:

    introJs.setOption('buttonCls', 'btn');