Visibility spreads. Still needs testing and an image for invisible.
Adds tests and code refactoring to GridModel for visibility.
Infection implemented. Still requires testing, and some code clarity work.
Updated the cell view's "not visible" image name to "fog", and made two style tweaks.
Merge branch 'develop' into feature-visibility
Adding tests.
Adds tests for infection initialization, spreading, and clearing in the model.
Image initialization for CellView refactored to take a CellModel.
Merge branch 'feature-visibility' into feature-infection
Small style corrections.
Merge remote-tracking branch 'origin/feature-infection' into feature-infection
Merge pull request #29 from zsweedyk/feature-visibility

Feature visibility
Small style nit corrected.
Style nits in testing file.
Merge pull request #31 from zsweedyk/feature-infection

Feature infection
Merge branch 'develop' into feature-spam-games
Fixed merge conflict (and added small comments).