I'm a little worried about whether using email rather than slug is appropriate for referring to badge instances, and that I may have created a monster.  It's very useful for the `GET /systems/:slug/instances/:email` route, but since badgekit-api-client as currently designed treats a certain model (in this case BadgeInstances) as having a single identifier that is used in all routes, this ended up leading me to use the email address in these routes as well.

I'm not entirely sure this is disastrous... it basically makes it so that the badgeInstance slug is used as the identifier in public routes, and the email is used as the identifier in secure routes.  Which might be ok.  I'm not sure though.