PROTON-196: Enable XML formatted output from python tests for better intergration with Maven/Jenkins

Also:

* Junit xml output now uses <skipped/> element to indicate a skipped test
* Junit xml output now includes message and type attributes on <failure/> element (Maven surefire module requires these).

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1442085 13f79535-47bb-0310-9956-ffa450edef68