need a spec for the exit-list format

The visitor blog post suggests that we want a python version of visitor. But the exit-list format isn't documented. It looks mostly straightforward, but things like "what time zone is that" and "what do you mean by laststatus" are unclear to me.

We also want people to be able to import these files into their own databases, e.g. if we give the files to gmail so they can annotate your logins with "(Tor)".

Giving them a walk-through of what they're seeing, and how they should interpret it, will increase the odds that they get around to using the information.

Once we have this spec, where should we put it? On the visitor howto? On the data.html page?