generate model group_user and migrate create_group_user
rake db:migrate create_group_user
setting group and user 多對多資料庫關聯
User 必須是社團成員才能發表文章
setting quit and join group in models/user.rb
setting quit and join group in controller/groups.rb
setting quit and join group in routes
setting quit and join group in views/groups/show
使用者建立討論版時自動成為討論版成員
如果不是 group 創辦人，就不會顯示 edit group 按鈕
Merge pull request #6 from sdlong/chapter6

使用者可以加入 / 退出社團