Art assets

This pull request mainly adds Michael's textures to the project.

In addition, I did the following:
* Created gray versions of the platform textures to represent breakable platforms
* The star was changed to a coin instead so that I don't have to change the Fixture object associated with the Treasure object
* A background was created
* Page 3 of the How to Play screens was updated to display the new textures
* The example screenshot in page 4 of the How to Play screens was changed
* The toolbox uses Michael's platform textures now (note: may cause merge conflicts)

Also, there are two versions of the example screenshot used in page 4: one without the background, and one with it. If you want, feel free to tell me which one you prefer :)

One thing to note though is that the background exposes a lot of ugly white pixels around the edges of the game objects, but I really don't want to try to get rid of those right now...

I think that's it for now.