Add the function for login from third party services

1. This PR only contains the login by Weibo and it's configurable, the example is in application.yml.example.
2. If user register by identity,  it can also connect with third party service(Weibo).
3.  If user only login from third party service(Weibo), he/she should could setup email.
4.  If user only login from third party service(Weibo), he/she should could also can setup the password for his/her account.
5. User can unconnect fromt he third party service(Weibo).
6. The architectural make people very easy to add any other third party service for login.

这里应该有个before_filter检查identity是否已经存在? 否则用户可以通过直接访问url绑定很多个identity，虽然不知道实际会有什么用...