[WIP] Refactor colors

[adamaaronson]

🚨 This is a breaking change, affecting the parameters when defining shapes and setting a scene's background color. 🚨

Addresses issue #449.

Done:

• Created colors module to represent RGB colors
• Refactored Shape colors, Scene background color, and Text color to use the new Color class
• Fixed example code and test code to use the new Color class
• Write tests for colors

Next steps:

• Add more ways to define colors (by hex? by name?)