Switched dependency from PIL to Pillow.
Slimmed down file headers.
Moved main files out of simsteg subdir, turned that into a package.
Simplified unnecessary error-handling.
Added tests dir. Added internal format tests.
Added shader tests.
Removed unused argument from invert shader.
Minor formatting change.
Added tests for sample intensity shaders.
Resolved discrepancy between github history and my local history.