Merge pull request #234 from liuchuangww/user

User module in process
User module front-end in process
Merge pull request #235 from sexnothing/user

User module front-end in process
Added create/add APIs to Pi::db()->where()  for predicates
Merge branch 'develop' into user
Refactoring message module
Updated user data API to support multi-uids for get/delete;Fixed potential errors for int value
Merge pull request #236 from jixingyu/user

Refactoring message module
Mute log message for sitemap
Merge branch 'develop' into user
Cleaning up message API: private message are not allowed to log; application logic should not be handled inside APIs
User module in process