Added new reporting plugin that produces junit-like xml output.

Written for/tested with Jenkins, but may be useful for other CI servers too.

Plugin structure based on xml_tests_report.rb -- couldn't get Jenkins to recognise it's Cppunit-like output. Also made the following changes to xml_tests_report:
* Moved xml_tests_report.rb into lib subdir to match new plugin directory layout and squash build warning.
* Corrected mismatched tags (<FailedTests></Test>)