Add the config parameter SendAttachment to send attachments.
 When SendAttachment is true, attachements will be created with size specifed in doc_size.

Change-Id: I24433355237e92c2686e8c21fe49b791348ed4d5
Reviewed-on: http://review.couchbase.org/38369
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Li Yang <lichenyang2@gmail.com>