Add first model
Merge remote-tracking branch 'upstream/master'
Add user admin site

- It seems that registration/autodiscovery is not working for user yet.
Add customized autodiscover and registration
Add activation callback and user perm change

- User permission change is essential for leveraging django admin site
- activation callback allows adding new default group
Merge pull request #14 from gary-pei/master

Add customized AdminSite, enable callback after account activation, change permission illustration