Support display names for roles

[j4mie]

For displaying roles to users of the application. Example:

You have received a new message from {{ user.get_profile.full_name }}. This user is an {{ user.role.display_name }}.

(display_name might not be the best naming..)

Should work just like model choices. User role names can (optionally) be tuples:

USER_ROLES = (
    'manager',
    'moderator',
    'client',
    ('admin', 'administrator'),
)