[logback-dev] System property configuration

All~ My company StreamBase (www.streambase.com) is planning on using slf4j and logback for its logging solution. However, we have needed to extend it to provide slightly more flexible default configuration. These are important to us as they provide us with needed flexibility in changing the log level easily for various debugging and testing situations. The attached patch adds support for some basic configuration via system properties. Including properties to control the following options: - config file - log level - use a console plugin (and port) Suggestions or improvements welcome, Matt