Linux: added project part for QtCreator.

Allows easily setup cocos2d on Linux in QtCreator IDE. User can just add 2 strings to `*.pro`, example:
```c++
COCOS2D_MODULES += extensions box2d CocosDenshion
include(/home/sergey/Applications/cocos2d-x/cocos2dx/proj.linux/cocos2dx.pri)
```

I can write wiki page about setup steps, if needed.