Assets can now be loaded from Json files (experimental threaded version)
GameWindow::draw now supports sf::RenderState parameters
Major resource system rework:
> now supports shader
> memory is now easily managed thanks to unique_ptrs and template/tag dispatching metaprogramming
> texture loading != image loading (if you added a texture before, it would also automatically add an image)

* syntax can be improved
* some "loadX(...)" methods are missing
Added sf::RenderState parameters to draw
Added UtilsJson to main header file, and added asset-loading from json files