Use a globally accessible WitchBlastGame object

- Add game() function, which returns the
  single instance of WitchBlastGame.

  This way, we can avoid the inconvenience of having to pass
  a WitchBlastGame reference to every object that wants to use it.

- Replace every WitchBlastGame reference with game().
Merge pull request #3 from crumblingstatue/master

Use a globally accessible WitchBlastGame object