add z-index to prevent wrong positioning of elements especially in Safari

I implemented your fullscreen plugin here: http://pro.mapsmarker.com/?p=11
For firefox, IE + Chrome this works fine - with Safari nethertheless some objects "shine through the fullscreen map" (see image attached). Fixed this by addint z-index.

![mapbox-fullscreen-zindex-issue-safari](https://f.cloud.github.com/assets/158669/272275/d670d0a2-8ffe-11e2-9fc8-2e2bf6055ff4.jpg)