[Jglogging-development] Switching Logger to be an abstract class, and added Logger.getLogger(...)

[Yannick M. <yan...@fa...>]

Oh, well...

I prefered to keep Logger as an interface, but the extra burden it adds
of having to LogManager.get().getLogger() instead of Logger.getLogger()
is not worth it.

So Logger is now an abstract class, and Logger.getLogger() functions
have been added it