New header image

This pull includes 2 new styles of header images : a basic one, and a cubic one. Vote for the one you prefer.

![both_main](https://cloud.githubusercontent.com/assets/7883281/4969414/6c32cd04-685f-11e4-9564-2536bc8a6c8a.jpg)

In-game preview for the basic one : https://mediacru.sh/hxnE0yYP-a3e
In-game preview for the cubic one : https://mediacru.sh/4xB5bbwHDtkC