Define z-index for project-box's flag and content divs

![screen shot 2014-07-30 at 5 18 56 pm](https://cloud.githubusercontent.com/assets/667753/3755658/fcf98a96-1826-11e4-84dd-afe452d305a6.png) ![screen shot 2014-07-30 at 5 20 03 pm](https://cloud.githubusercontent.com/assets/667753/3755662/054a8380-1827-11e4-8f6b-5c77b86f52b0.png)

Fixes #445.