Feature: TLS support

[ysamlan]

We're using it behind load balancers / gateways that terminate SSL, but for anyone who isn't, it'd be good to have a form of NewGroup like NewGroupWithTLS (or just some extra struct fields TLS folks can set after NewGroup) if they want the net/http.Server that gets spun up to support TLS directly.

I probably won't implement this myself since we aren't likely to need it at Localytics, but I don't think it'd be too much creep to add if someone wants to do the work (as long as the basic non-TLS case doesn't get any harder to use because of it).

(I think this is only needed for the main :8080 service, not the :6060 pprof debug server -- if that's exposed on the wider internet you have bigger problems than TLS).