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 23 08 am](https://f.cloud.github.com/assets/696982/548813/fda3b7be-c2eb-11e2-9191-2c189c7ba833.png)