Add background graphics

[nicolasbock]

We should have some background graphics. One way to do that would be to introduce additional moving layers behind the game layer that are moving slower to give the illusion of depth.

[nicolasbock]

@mkotamies Since you worked on the favicon I'll ask you 😄 Do you have any suggestions or ideas how to do that? What we are looking for I think is some background graphics that's scrolling at a different speed compared to the floor to give the impression of 3D. We could even think about adding several background layers that all move at different speeds to enhance this 3D impression. What do you think?