Audience level now shown for tutorials.

Just added the audience level to let users get a quick feel for what tutorials might be of interest.  Refers to issue https://github.com/PyCon/pycon/issues/261

![audience-levels](https://cloud.githubusercontent.com/assets/470417/4228907/bae1f7c4-3964-11e4-9fe5-ba61b0a6b747.png)