get_download, returns better message, meaningful metadata.

get_download returns only one object ever (unless we make it do multiple
calls to the API), so it can have a unique message, that now indicates
which site was returned, to act as a check for the user.

Also, \code{site.data}, \code{pi.data} and \code{meta.data} are all
metadata, so I've moved them into a list object.  I'm keeping
\code{sample.meta} out for now because it relates directly to the actual
sample data.