Adds os-server-groups

This commit adds support for the os-server-groups extension. This allows
users to create scheduling policies for launching instances.
diff --git a/openstack/compute/v2/extensions/servergroups/urls.go b/openstack/compute/v2/extensions/servergroups/urls.go
new file mode 100644
index 0000000..074a16c
--- /dev/null
+++ b/openstack/compute/v2/extensions/servergroups/urls.go
@@ -0,0 +1,25 @@
+package servergroups
+
+import "github.com/rackspace/gophercloud"
+
+const resourcePath = "os-server-groups"
+
+func resourceURL(c *gophercloud.ServiceClient) string {
+	return c.ServiceURL(resourcePath)
+}
+
+func listURL(c *gophercloud.ServiceClient) string {
+	return resourceURL(c)
+}
+
+func createURL(c *gophercloud.ServiceClient) string {
+	return resourceURL(c)
+}
+
+func getURL(c *gophercloud.ServiceClient, id string) string {
+	return c.ServiceURL(resourcePath, id)
+}
+
+func deleteURL(c *gophercloud.ServiceClient, id string) string {
+	return getURL(c, id)
+}