Move engine-api client package

This moves the engine-api client package to `/docker/docker/client`.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container_export.go

@@ -0,0 +1,20 @@
+package client
+
+import (
+	"io"
+	"net/url"
+
+	"golang.org/x/net/context"
+)
+
+// ContainerExport retrieves the raw contents of a container
+// and returns them as an io.ReadCloser. It's up to the caller
+// to close the stream.
+func (cli *Client) ContainerExport(ctx context.Context, containerID string) (io.ReadCloser, error) {
+	serverResp, err := cli.get(ctx, "/containers/"+containerID+"/export", url.Values{}, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	return serverResp.body, nil
+}