Added support for os-floating-ips extension

This commit adds support for the os-floating-ips extention. This allows
users to allocate and deallocate floating IPs as well as have instances
associate and disassociate floating IPs in a nova-network based cloud.
diff --git a/openstack/compute/v2/extensions/floatingip/urls.go b/openstack/compute/v2/extensions/floatingip/urls.go
new file mode 100644
index 0000000..54198f8
--- /dev/null
+++ b/openstack/compute/v2/extensions/floatingip/urls.go
@@ -0,0 +1,37 @@
+package floatingip
+
+import "github.com/rackspace/gophercloud"
+
+const resourcePath = "os-floating-ips"
+
+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)
+}
+
+func serverURL(c *gophercloud.ServiceClient, serverId string) string {
+	return c.ServiceURL("servers/" + serverId + "/action")
+}
+
+func associateURL(c *gophercloud.ServiceClient, serverId string) string {
+	return serverURL(c, serverId)
+}
+
+func disassociateURL(c *gophercloud.ServiceClient, serverId string) string {
+	return serverURL(c, serverId)
+}