Added images for fog
Merge pull request #30 from zsweedyk/feature-fogImages

Added images for fog
Updated the cell view's "not visible" image name to "fog", and made two style tweaks.
Merge branch 'develop' into feature-visibility
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