mv README.md to wiki
1. add checkouts/update integ test. issue found when checkout/search, when
do: checkout = Checkout.find_by(id: checkout_id), the checkout.group_id
is nil. Very strange. Minghe will check.

2. fix typo in checkouts_controller.rb
Merge branch 'hbzhou' into dev
Merge branch 'dev' into minghe_dev
fix the checkout update issue
    1. typoe, group_id ---> group.id, json --> jsono
    2. checkout should be save when update it
Merge branch 'dev' of https://github.com/us4us/seed into mia_dev
remove some images
Merge branch 'dev' into mia_dev