New header image

This pull includes 6 new styles of header images. Vote for the one you prefer.

![both_main](https://cloud.githubusercontent.com/assets/7883281/4985319/441cff74-692c-11e4-86f2-48f656b152ef.jpg)

In-game preview for the basic one : https://mediacru.sh/hxnE0yYP-a3e
In-game preview for the basic one (style2) : https://mediacru.sh/g8G0cZX5yr6j
In-game preview for the cubic one : https://mediacru.sh/4xB5bbwHDtkC
In-game preview for the cubic one (smaller cubes) : https://mediacru.sh/54Lmn02G2Lsn
In-game preview for the letters-land one (style 1) : https://mediacru.sh/T9bW-d9KnZDx
In-game preview for the letters-land one (style 2) : https://mediacru.sh/SX6pvCYVZqYu
In-game preview for the letters-gameland (style 1) : https://mediacru.sh/CoXjwVwGryrp
In-game preview for the letters-gameland (style 2) : https://mediacru.sh/mzkEQC1COS6l