add throwable information to xml layout if present

[chbiel]

Include the throwable information, that are present in the LoggerLoggingEvent, into the xml layout if present

[kategray]

The code looks fairly clean, but I'd like to see unit testing to ensure that the output is what it should be.

@rgoers I can put one together if you'd like, but I'd suggest not approving this until there's a unit test for it.

[rgoers]

OK. Got ahead and create the unit test.