Adding CSS Class for Series

We should add in a CSS class for the series to the `<aside class="wp-post-series-box">` that way we can style it differently for each series.

![adding-series-css-class](https://f.cloud.github.com/assets/1065372/1827783/d0283d96-7240-11e3-9edc-0401dd3e8cb9.png)