Provide lightweight alternative implementations: log4j-simple and log4j-nop

To encourage usage of Log4J 2 API as a ubiquitous logging API for Java/JVM, we should provide some lightweight alternative implementations that can be used instead of log4j-core:

• log4j-simple which uses SimpleLog without any error or warning message.

• log4j-nop which silently discard all logging.

This would be useful when libraries/frameworks use log4j-api. Then users of such libraries/frameworks who are not interested in a comprehensive logging system will have an easier time. It will also help usage on "odd" platforms like Android where log4j-core can be problematic.

(This is inspired from SLF4J.)