Better debug output

[acoburn]

The test suite generates a nice HTML report summary, but there is little indication of why a particular test fails. Adding debug output is no help if the test case encounters an exception (e.g. if it can't parse a link header as described in #232). There is an output log file, but that isn't particularly helpful in most cases (no stack traces are output). The RDF-based output file has some better diagnostic information, but the format is incredibly hard to read. Some sort of debug output switch would be much more helpful. And if there already exists such a thing, please document it in the README.