Add roles for Persona and implement permission logic
Add Persona validation specs
Fix specs to treat Persona role
Update the principle strategy of tests and models

1.  Disable db strategy of `is_staff` and `is_superuser` and replace
    these with role based strategy
2.  Rename some unclear permission names of persona model
3.  Make the permission strategy clear. Adam is a superuser and Seele
    can change the user's role thus only Seele can be a Adam.
Merge pull request #26 from kawazrepos/feature/user-group

Implement new Persona role system