Add `wicked_pdf_url_base64`.
Add `wicked_pdf_url_base64`.

Using URLs to reference images can cause a lot of problems with wkhtmltopdf, particularly when used in the header and footer. It produces errors such as "Too many open files" as well as buffer/stack overflows.

A good solution to this is providing the image encoded as base64. This helper method takes a URL of an image, opens the URL, reads the image data and encodes it to base64.

If the URL does not have a successful response (`response.is_a?(Net::HTTPSuccess) == false`), it will log a warning and return `nil`. We could optionally make this raise but that should be a project-wide change.

TODO:

- [ ] Better placement (since this is not technically part of the asset pipeline)?
- [ ] Better naming?
- [ ] Tests.

mileszs/wicked_pdf,bonekost/wicked_pdf