Added a test reporter for JUnit XML

Taken from https://github.com/chenkelmann/junit_xml_listener and slightly improved.
Activating is now just a matter of setting `testReportJUnitXml` to true.
This not only brings a very handy feature (hudson/jenkins support that syntax),
but gives more options for testing the SBT test infrastructure.
For instance checking that the test output is well segregated during a parallel
test run is just a matter of checking the report output.