Add "date created" metadata to experiments

[kalekundert]

I think I currently rely on file modification times to figure this out, but that's not reliable. Now that I have a metadata directory for each experiment, this seems like a good thing to use it for.

That said, this does make it less tractable to create an experiment directory by hand. That ship may have already sailed with the existing experiment ID metadata, though. Maybe I should provide an exmemo expt init command that turns an existing directory into an experiment.