* SMTP notify enhancements: HTML emails and customization options
- Send full HTML emails, with or without inline attached images.
- Custom `timeout`.
- Custom `product_name` identifier for the `FROM` and `X-MAILER` email headers.
- New HTML email test
* `sender_name` instead of product_name
- Change `sender_name` instead of `product_name`.
- No changes in `X-Mailer` header.
- `From` header as always unless you define the new `sender_name` parameter.
* Added optional embedded image attachments to notify.smtp.
Also restructured a bit to minimize code duplication and add some tests.
* Fixed formatting errors.
* SMTP cleanups thanks to code review.