Order receipt email should include the transaction date

I realized today while thinking about the "place order on another date" feature of order creation that our order receipt email doesn't actually include the date the order was placed or payment was made. This is a problem in general, not just for late or delayed placement of admin orders, as the timestamp of an email being sent or received may not correlate to the time of the order or payment.

For reference, here's the order receipt email as sent by a clean installation of Commerce Kickstart:

I think we have three options we need to choose from:

1. Add the order placed date to the email.
2. Add the payment placed date to the email in the payment section.
3. Both of the above.

I tend to think option #1 is our safest bet for core. Aside from the reality that Order doesn't depend on Payment, which we could make accommodation for like we did to get shipping details in there, we also don't know if all payment methods on a site would even support or properly be represented as being placed on a given date (e.g., bank transfers with multi-day approvals, checks, invoices, etc.).

Thus, I'd propose something like:

This changes our current header a bit similar to other receipts I see from retailers I visit that email copies of receipts. Another option would be to put it in the footer near the order total, though I think it deserves greater prominence.