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/requests.go b/openstack/blockstorage/extensions/volumeactions/requests.go
index d2a2c79..1aff494 100644
--- a/openstack/blockstorage/extensions/volumeactions/requests.go
+++ b/openstack/blockstorage/extensions/volumeactions/requests.go
@@ -50,6 +50,15 @@
 	return
 }
 
+// BeginDetach will mark the volume as detaching
+func BeginDetaching(client *gophercloud.ServiceClient, id string) (r BeginDetachingResult) {
+	b := map[string]interface{}{"os-begin_detaching": make(map[string]interface{})}
+	_, r.Err = client.Post(beginDetachingURL(client, id), b, nil, &gophercloud.RequestOpts{
+		OkCodes: []int{202},
+	})
+	return
+}
+
 // DetachOptsBuilder allows extensions to add additional parameters to the
 // Detach request.
 type DetachOptsBuilder interface {