write better version-uptake measurement tool

[warner]

I've got a rudimentary munin plugin that takes the client-version data from #3 and attempts to make an area chart of how many clients were using which versions. My intention was to sum the usage numbers in a rolling window, spread over the last hour or so. But I did it wrong, or it's interacting with the blurred timestamps in the logs somehow, and I wind up with weirdly-truncated graphs like this one:

The lack of data for the last 12 hours is issue #13, and unrelated to this one. This bug is that the graph grows from zero at the start of each hour, to some peak, then resets back to zero again.

I suppose having all the timestamps rounded down to the nearest 60 minutes might explain this, in which case I should really be summing over the last 12 hours or so.