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
Merge pull request #237 from liuchuangww/user

User module in process
user join selection
user join selection
user join selection
Completed setting avatar
user join selection
Add robot.txt
Merge pull request #238 from linzongshu/user

Completed setting avatar
Merge pull request #239 from voltan/develop

Add robots.txt
Merge branch 'develop' into user