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/results.go b/openstack/blockstorage/extensions/volumeactions/results.go
index f9e8120..b5695b7 100644
--- a/openstack/blockstorage/extensions/volumeactions/results.go
+++ b/openstack/blockstorage/extensions/volumeactions/results.go
@@ -7,6 +7,11 @@
 	gophercloud.ErrResult
 }
 
+// BeginDetachingResult contains the response body and error from a Get request.
+type BeginDetachingResult struct {
+	gophercloud.ErrResult
+}
+
 // DetachResult contains the response body and error from a Get request.
 type DetachResult struct {
 	gophercloud.ErrResult