(#171) - multipart api

Introduce `db.multipart` api endpoint for making multipart/related requests:

    db.multipart.get(doc, attachments, [params], [callback])
    db.multipart.insert(doc, attachments, [params], [callback])

Note: the `db.multipart.get` response is an unparsed `Buffer`.
Merge pull request #172 from jo/multipart

(#171) - multipart api