Provide an indication of the health/performance of each exit location

From #79 (closed):

@gk: Yeah, it might be neat if we could give some performance indicator and I like the mock-ups you made. The number of relays alone does not matter so much here, though, as you could easily imagine 50 exits being in a country where all of them are configured to just relay 100KiB/s or so, which would be very annoying. We should therefore look at the exit capacity (as well).

If that's the way to go let me know and I could look at the data we have for exit capacity per country and we could then think about proper thresholds for your poor/normal/good perf indicator.