Update LoggerStream API

I've got some ideas on how to improve the LoggerStream idea that I added a little while ago. The main thing I'd like to do is extract an interface from it, rename the default implementation to SimpleLoggerStream (part of the SimpleLogger stuff), and allow log4j implementations to specify a different implementation if desired.

In doing this, I'm not sure where specifically I'd prefer the getStream methods to be. Right now, it's in Logger, but really, it could be in LoggerContext instead. I don't think I should be required to get a Logger just to get a LoggerStream.

Now if only the java.io package used interfaces instead of classes. This would be so much easier to design!