Allow the SLF4J loggers to be @Injected

SLF4J is being used by 15 Apache projects and counting[1] is a good API for logging, has many implementations and Sisu/Guice has support for @Injecting them which makes it convenient in extensions and plugins.

The MavenCLI needs to be cleaned up and when a default implementation is picked the logging mechanism can be simplified and all Plexus logger and SLF4J logger instsances can all be routed through the same mechanism.

[1]: http://slf4j.org