2.1 rails new demo_app
2.1 Use sqlite3 gem in only development mode and add pg gem for production mode
rails generate scaffold User name:string email:string
2.2 rake db:migrate
2.3 rails generate scaffold Micropost content:string user_id:integer
2.3 Add validation for Micropost.content (max-length: 140)
2.3 Add associations (user has many microposts)
2.3 Add associations (micropost belongs to user)
Merge branch 'chapter02_demo_app'