Mailers

This adds some emails you can opt-in to sending to your customers.

Opt into them like this:

```ruby
Payola.configure do |config|
  config.send_email_for :receipt, :admin_receipt
end
```

Possible emails include:

* `:receipt`
* `:refund`
* `:admin_receipt`
* `:admin_dispute`
* `:admin_refund`
* `:admin_failure`

`:receipt` and `:refund` both send to the email address on the
`Payola::Sale` instance from the `support_email` address. All of
the `:admin` messages are sent from and to the `support_email`
address.

This also refactors how background workers work. Specifically, how you need to set up a `background_worker` lambda. See README for details.