add smart app banner to promote ios app

Add an HTML element to promote ownCloud's iOS app.
This banner will only be displayed once until the user presses (x).

![IMG_1508](https://f.cloud.github.com/assets/1250540/97618/f3c09b5a-6700-11e2-85a9-3cd7cad81022.PNG)

please review @DeepDiver1975 @karlitschek @jancborchardt