Update seed data and relevant models

  * Adjust validations in `item`, `category`, and
    `item_category` to use `inverse_of` for
    validating presence. For discussion on topic:
    https://github.com/rails/rails/issues/5178#issuecomment-11529392

  * NOTE: There are several comments left in
    intentionally. Please clean them up. They are
    there for comparison and to answer questions
    you may have.

  * The `seeds.rb` file is destructive. It will
    remove existing data from the database when
    run. Read the code to delete these two lines
    if this is not desired behavior.