add optional slide element class

I had some trouble when integrating this with an ember.js application due to the container just using jquery's children method on the container. The problem I was having is visible in the screenshot

![worthit](https://cloud.githubusercontent.com/assets/3799709/3374672/2e228c94-fbc1-11e3-8b7c-f15a6a13350a.png)

I added an optional slide element option that is passed to children instead of without any arguments