Add UIDocumentInteractionController as PDF viewer.

Re-introduce the PDF viewer to the showcase.

![schermata 28 nov 2013 17 34 25 simulatore ios](https://f.cloud.github.com/assets/953519/1641134/fe0e1376-584a-11e3-8ccf-9ad6607eff8b.png)
![schermata 28 nov 2013 17 33 52 simulatore ios](https://f.cloud.github.com/assets/953519/1641135/fff6f54a-584a-11e3-90b9-ec3c00993c56.png)