add group integration test. issue found: new group can not be saved.
fix the save issue
finish the checkout create action and move wallet_in_group to helper
add message field to return json even its status is SUCCESS
add empty message field when SUCCESS
Add test for group APIs and fix issues I find.
- changes of groups_controller.rb
1. after create user, user need to be saved again because the creator, wallets...of user are updated.
2. fix typo:  update_inf -> update_info
3. fix typo: grou.creator_id -> group.creator_id

- changes of helpers/checkouts_helper.rb
1. fix typo: group_id -> group.id

- change of routs.rb
1. add route for /groups/show

- change of test/integration/group_api_test.rb
1. add test case for all group apis
2. right now only group dejoin doesn't work correctly.
Merge branch 'dev' of github.com:/us4us/seed into dev
Merge branch 'dev' of github.com:/us4us/seed into dev
fix following issue
    1. user can dejoin himself when his wallet in group is aout 0.0
    2. user user.groups.delete(group) to remove a group from his groups
    list
    3. update the api doc