add type column.
Start refactoring
Fix storng params.
Use iterated params for artist levels.
Add tests to make sure STI is working.
more tests to make sure STI is working.
Refactoring.
Add html and logic for karel builder.
Add test for maze/karel distinction in maze builder.
Merge pull request #516 from code-dot-org/STI-for-levels

Sti for levels