This updates the PMD test suite to add both real-world simulation and

integration tests. The tests have been expanded to test for files

without errors, invalid ruleset configurations, and errors processing

source files (syntax errors).
This uncovered a handful of issues, mostly in the processing failure or

invalid ruleset cases, which have all been fixed. This code is more

bullet-proof, and generates more helpful error messages in the review

request.
We're now using the JSON output, rather than the CSV output, which is

more useful and safer to parse. There shouldn't be any compatibility

issues with this.
We're also explicitly turning off report caching, since we don't want to

preserve that information anywhere.

All unit tests pass on Python 2.7 and 3.x.