XML reporter

This PR adds a JUnit XML reporter. It uses the same JUnit format as Hudson, which may be slightly different than what Ant generates. Output is written to a `report.xml` file when the test runner finishes. The reporter works with Node.js, both the node client and runner.
Since there is not more than one XML reporter, I’d prefer to just make this part of the reporter itself, at least until there is demand for it to be broken out