Add JSONLayout class

[bblough]

This layout will format the LoggingEvent information such the date,
logger, level, message, and location information, as well as the values
in the MDC and NDC, into valid JSON. This is useful for consumption
by elasticsearch or other log tools.

[ams-tschoening]

Thanks for the work. Would be great if you can look at the minor changes I mentioned. Besides that, did you consider something like pretty-printing of the output or do you think it's not worth it? Might it even break your use case?

[bblough]

For my use case, I think single-line log entries are preferred. But I can see how other use cases might benefit from pretty-printed output. I'll add it as a option.

[bblough]

Pretty-print option added. All changes pushed.

[ams-tschoening]

Thanks for changing things, but I additionally ran into problems getting the new layout to compile in case the WCHAR_T-API is used and your tests failed on my Windows as well. The first problem could easily be changed by using LOG4CXX_STR everywhere and the latter by using LOG4CXX_EOL instead of \n. HTMLLayout seemed to have done the same.

Would be great if you could have a look at my newly created branch containing your PR and my additional fixes, if things still work for you.

https://github.com/apache/logging-log4cxx/tree/ghpr_13

[bblough]

Ah, sorry about that. Thanks for fixing it. I've just compiled and tested here and it still works for me.

[ams-tschoening]

That's good news, so I merged everything to master. Thanks for the contribution.