Make the role in relations optional, with a test to make sure it is. Also start movin...

Make the role in relations optional, with a test to make sure it is. Also start moving the errors reading the xml to exceptions, thus making it possible to give meaningful error messages, when bad xml is sent (More work is required on this including doing the same for nodes and ways). With the latest gems update it seems that the lib xml handling was broken, using the newer method. Adding the content type for the exceptions.