Added XML matchers for TestResponse (similar to Specs) to test if the XML of the response contains certain nodes

Ticket: https://www.assembla.com/spaces/liftweb/tickets/640-add-xml-matchers-to-testresponse

Tested each of the 8 operators for success and failure by locally modifying ESME's TwitterAPI test:

http://svn.apache.org/repos/asf/incubator/esme/trunk/server/src/test/scala/org/apache/esme/api/TwitterAPITest.scala