Banea Dominios

Creating logic for banning specific email domains.
I've been reading about using default scopes as an evil/dangerous thing because if you forget that you have it and don't explicitly use `.unescoped` (and you have to do ir right) will cause unexpected behaviors and headaches trying to figure things. 

In this case I see no problem about it since we have an unique attribute and will be just default order, but I generally would avoid using `default_scope`

Articles:
http://www.databasically.com/2011/02/07/default_scope-use-cases-caveats-and-work-arounds/
http://rails-bestpractices.com/posts/806-default_scope-is-evil