Vmitnick/crema headlights

This logs an additional headlights event when assets are generated for a PSD where the user used Crema to set up generator, or for a PSD where the user didn't use generator, so we can compare the two.

To avoid having an even for every asset generated, I'm currently limiting it to a single one of each of these events per PS session, but might change that.
D'oh. I'll remove that. I added it while trying to figure out why something wasn't working right, and I had seen an (apparently out-of-date) error in a log file complaining that document was undefined here.

Don't merge yet, though, Ian - I'm going to play with the throttling a bit and should have a new change up soon.