Add documentation section on debugging tests

Is this really a topic to be included in the usage documentation? I would rather say it is a development topic. For regular test debugging, I simply launch the tests from the IDE directly, i.e. don't bother to run/debug a Maven JVM. Andreas' question was about debugging Surefire not about debugging his tests, that's a different use-case.

Therefore it might be better to create a general site article "Debugging Maven" somewhere in the developer area. This article would then contain extra notes on Surefire debugging.

Adding documentation on debugging Maven would probably be a good idea, too. I think tips on how to use maven.surefire.debug is a good idea regardless, so I checked in revision 615426.