Support PET entity revisions

Feature request for making the PET entities revision-able. Personally I have no idea how much work this is, but I can tell you how it would be useful.

Currently I'm using PET to manage my content inside my emails. They are sent via mandrill module which wraps it around a mandrill wrapper.

We have a content editor, which tweaks the emails a couple times a week and we track conversion differences based off that via click rates in Google Analytics.

If I had PET revision IDs, I could auto-add that into my template google link campaigns, to automatically generate revisioned campaign links to track and see what works best over the long run.

Additionally it would allow us to revert back to previous version if those worked best.

This could kind of be done as well if the PET entity had an "last updated" timestamp.

Just a thought and a feature which would be useful to me.

I personally don't plan on creating this, but I figured it might perk up someone else's interest.