create flavor (#257)

* create flavor operation

* accept 200,201 status codes

* unit test
diff --git a/openstack/compute/v2/flavors/urls.go b/openstack/compute/v2/flavors/urls.go
index ee0dfdb..2fc2179 100644
--- a/openstack/compute/v2/flavors/urls.go
+++ b/openstack/compute/v2/flavors/urls.go
@@ -11,3 +11,7 @@
 func listURL(client *gophercloud.ServiceClient) string {
 	return client.ServiceURL("flavors", "detail")
 }
+
+func createURL(client *gophercloud.ServiceClient) string {
+	return client.ServiceURL("flavors")
+}