#4 Referer url's grouped by domain

[Simon Mikkelsen]

The statistic of referer urls, should be able to be shown
grouped by domain; e.g. like a separate table or list.

If possible this should be modeled to be reuseable on all
statistics on which it makes sense.

This request is motivated by a user who have up to
12000 uniqe referer urls, from a small number of domains.