Fix issue: checkout has no attandees info.
1. Add CheckoutAttandee model to record attandees for each checkout.
2. Update checkout index controller to return full checkout info
including attandees.
3. Update fixture and controller test to test change #1 #2.
Merge branch 'dev' into hbzhou
Update interface of attandees of checkout: "attandees" : [{"id" => xx},
{"id" => yy}]