redo streaming interface

now returns an io.WriteCloser/io.Reader, which is more client-friendly.
previously you'd basically need a goroutine.

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotallabs.com>
send response for StreamIn, close writer when done

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotallabs.com>