Python 3 support + PEP 8

Hi Jessica,

First of all, I want to thank you for everything you do for advancing Python and CS; it is inspirational ☺

Now that my 6 y.o. daughter Ilil is familiar with the letters of the Latin alphabet, I want to teach her programming. Your ColorWall seems like a good place to start, since she really like drawing and colouring repeating patterns (this way she will be able to program the computer to draw colourful patterns). Since I think it is better to begin with learning Python 3 (as opposed to Python 2), I updated ColorWall to support it (now it supports *both* versions, 2 and 3). Along the way I changed the styling of some lines in order to conform with PEP 8 better.

Please accept this pull request so other people will be able to use ColorWall with Python 3 as well.

Thanks,
Júda