Fix hotbar background

https://github.com/freeminer/freeminer/issues/178

You should replace games/default/mods/default/textures/gui_hotbar.png with 
![gui_hotbar](https://cloud.githubusercontent.com/assets/1615889/5556349/53b7eaba-8cfa-11e4-8980-a8c8b73c5999.png).

Some screens:
![2014 12 26_12 43 32 550410562](https://cloud.githubusercontent.com/assets/1615889/5556412/61e277c0-8cfc-11e4-9208-74c171a997da.png)
![2014 12 26_12 43 44 504426659](https://cloud.githubusercontent.com/assets/1615889/5556413/6535d926-8cfc-11e4-82a5-d98da8ee40a7.png)