Add function to get attachments

Attachment support is added recently, but there is no way to get the attachment, without first fetching the document. This patch adds a convenience function to get an attachment for a document.

Real issue at hand here is that we do not have access to the `serialize(type, id)` function. That function is needed to know the `_id` pouch uses internally to be able to fetch the attachment.

Maybe a better approach is to make the `serialize(type, id)` public API and let the developer implement its own methods.