Allow metadata (location, contact information) to be added to announcements

[soult]

It would be useful for the VolunteerGrid if users could add some metadata to announcements, e.g. storage node location, contact information or notes about the connectivity. This would probably also help to implement the idea in ticket #642.

IRC log (UTC)

[21:21]	<zooko> Ooh, I want a way to put metadata into the
announcement, next to the "nickname" so that we can, if we choose,
put geographic location in there.  :-)
[21:22]	<soultcer> I added a table at
http://allmydata.org/trac/tahoe/wiki/VolunteerGrid but metadata
for storage node announcements sounds easier to maintain and more
fun. Maybe you can add not only a location node but also a contact
node, in case we need to contact node owners.

[warner]

Yes! #466 (extensible introducer announcements) would make this pretty easy: the biggest challenge would be deciding the name of the dictionary key.