Adds ability for users to have multiple roles.

Adds new mapping table to link users with multiple roles. Tested and works, however, because it changes the ab_user table (removes the role_id column) and needs another table for role mapping, it breaks FAB when used with already existing database.

This is not ready to be merged in yet, but I did a pull request to keep comments and code in a central place. Hopefully I can add migration stuff and then we can do a merge if everything looks good. 

Migration should be easy (drop column, add mapping table and do insert based on user's role_id) but I'm not sure where / how to put that.