Bring CC into the Present!

• Migrate to python3
• Switch to recent numpy version
• Fixed serious network bug ("BuildPlayerClient does not have attribute 'pos'")
• Code cleanups
• Difficulty selection for GruntAssault mode
• Fixed crash if not sound card is available and you try to turn the sound off (Thanks John Lightsey)
• Fixed typos in documentation (Thanks John Lightsey)
• Improved documentation
• Backgrounds for all play modes
• Fixed bug in initialization of 3 and 4 player "river" games