generate migrate create_group_user and model group_user
rake db:migrate create_group_user
setting user and group 多對多資料庫關聯
User 必須是社團成員才能發表文章
generate quit and join group in models/user.rb
generate quit and join in groups_controller
generate quit and join group in models/user.rb
generate quit and join in views/groups/show
User is member of group that create himself
如果不是 group 創辦人，就不會顯示 edit group 按鈕
Merge pull request #7 from sdlong/t12089

實作簡單的 Account 後台機制