Add ActiveRecord
peg activerecord to 4.0.4 (last version working with Sinatra)
Use ActiveRecord to store data

- A database migration has been created for the todo_items table
- The database details were added to the top of app.rb
- That migration was run, creating the db/schema.rb file. The db/development.db file was added to .gitignore, since it's the database.
- The file models/TodoItem.rb was added to define an ActiveRecord model for the todo_items table
- The controllers in app.rb and the view in views/index.erb were modified to use ActiveRecord methods