Add BeginDetaching call for Cinder v2 (#90)

Per Cinder API, os-begin_detaching action needs to
be called prior to os-terminate_connection.
diff --git a/openstack/blockstorage/extensions/volumeactions/urls.go b/openstack/blockstorage/extensions/volumeactions/urls.go
index a61ee7d..a172549 100644
--- a/openstack/blockstorage/extensions/volumeactions/urls.go
+++ b/openstack/blockstorage/extensions/volumeactions/urls.go
@@ -6,6 +6,10 @@
 	return c.ServiceURL("volumes", id, "action")
 }
 
+func beginDetachingURL(c *gophercloud.ServiceClient, id string) string {
+	return attachURL(c, id)
+}
+
 func detachURL(c *gophercloud.ServiceClient, id string) string {
 	return attachURL(c, id)
 }