os-networks extension

This commit adds the os-networks extention. This can be used to view
details about the nova-network-based networks that a tenant has access
to.
diff --git a/openstack/compute/v2/extensions/networks/urls.go b/openstack/compute/v2/extensions/networks/urls.go
new file mode 100644
index 0000000..6966462
--- /dev/null
+++ b/openstack/compute/v2/extensions/networks/urls.go
@@ -0,0 +1,17 @@
+package networks
+
+import "github.com/rackspace/gophercloud"
+
+const resourcePath = "os-networks"
+
+func resourceURL(c *gophercloud.ServiceClient) string {
+	return c.ServiceURL(resourcePath)
+}
+
+func listURL(c *gophercloud.ServiceClient) string {
+	return resourceURL(c)
+}
+
+func getURL(c *gophercloud.ServiceClient, id string) string {
+	return c.ServiceURL(resourcePath, id)
+}