added seeds.rb and updated migrate files for t.belongs to

- closes #9 
- Added seeds.rb file
- updated migrate files for t.belongs_to rows
- added single table inheritance for questions, which is reflected in seeds.rb file
Can you put these in separate files? e.g. `attendance_question.rb`, `multi_choice_question.rb`, etc. Also, a true-false question is just going to be a `MultiChoiceQuestion` in the backend.