Rename model checkout_comments to comments.
Add picture column for model Comment.

Tests and seeds are updated also.
Add comment query API.

/comment for all comments
/comment/:id for comment with certain id.
create comments query APIs.

- /comments for all comments
- /comments/:id for comment with certain id
- /users/:user_id/comment for all comments created by certain user
Merge branch 'hbzhou' into dev