- rename 'group' and 'user' columns in group_memeber table to 'user_id'
  and 'group_id' respectively. The reason is that Laravel is giving me
 errors in unexpected places in migration. As 'group' and 'user' are
MySQL keywords and Laravel excape them with backtick(`) only on certain
occasions, we encounters errors when it fails to escape them.
You must have id column in every table in Laravel even for intermediate
join table. It won't work with composite primary key. I thus have to
drop the composite primary key and make a composite unique key. After
that add an id column.

- unit test for group member
- group member