Modify game hash method to be two-way and based only on id and group.

[jwir3]

This makes every Game object's getHash() method actually be a cipher which will return a combination of game id and group id. In doing this, it allows us to decipher the hash back to game id and group id, which sets us up to more easily fix #67.

Issue References:

• Fixes #84.
• Refs #67.

Development/Code Review Checklist

• [x] Documentation updated
• [x] Tests written for new features
• [x] Tests complete successfully
• [x] Formatting conforms to code style guidelines