Treat image attachments as binary blobs;tools/dump fetch attachments

The _all_docs view only returns attachment stubs and doesn't accept the attachments=true attribute so I changed the dump tool to fetch full documents in the main loop.  Also added guard check to mulitpart writer to not attempt to decode images and not bloat them with a utf-8 encoding.  There is probably a better way to detect binary mimetypes, if pointed in the right direction I can improve that check.