移动端访问aws的方案
* 使用 aws的root 用户帐户信息 ： -----权限太大，不合适
* 使用 aws的创建的IAM user的信息： ------无法轮换身份信息
aws的思路------------>利用token vendor machine方案
Anonymous TVM
Cognito
User Pool vs Identity Pool
参考：
https://aws.amazon.com/cn/articles/authenticating-users-of-aws-mobile-applications-with-a-token-vending-machine/
https://aws.amazon.com/cn/code/token-vending-machine-for-identity-registration-sample-java-web-application/
http://docs.aws.amazon.com/zh_cn/STS/latest/APIReference/API_AssumeRole.html
https://stackoverflow.com/questions/42521695/aws-sts-assumerole-do-i-have-to-provide-access-and-secret-keys
aws-sdk-ios-samples: https://github.com/awslabs/aws-sdk-ios-samples
http://www.jianshu.com/p/112438fb86aa