Charges

Behold!
Nit: It would be great to move the closing paren and the call to `exec()` to a newline.

Other nit: Is `aggregate()` a bit overkill here? I agree that we should suppress `_id`, but it feels like it would be simpler to write `Charge.find({ product_id: productId }, { _id: 0 })`. And I'm not wild about aliasing `stripe_charge_id` to `id` in the returned objects, as that's not done elsewhere.