Allow organizing notaries into groups

[daveschaefer]

Feature request via email: Let people organize notaries into groups, and have the client require responses from each group.

Combined with #97 this would let you do things like "require a reply from my personal notary and any other 3 notaries", or let you group notaries by country and require 1 reply from each country/area group.

Discussion thread on the mailing list: https://groups.google.com/forum/#!topic/perspectives-dev/szqkIJT3Ds0