basic roles

1. add roles to User and InstanceUser model

2. add two tests and refine existing tests, remove should statement ( http://betterspecs.org/#should )
I don't think we should modify schema.rb, otherwise when we re-run migrations, the order will be changed back. 

http://guides.rubyonrails.org/migrations.html
"They are not designed to be edited, they just represent the current state of the database."