See http://www.slf4j.org/manual.html#gradual for details
Please note that jcl104-over-slf4j requires a valid SLF4J
binding. However, since the binding can vary according to your
taste, the SLF4J requirement is NOT listed in this project file.
Javadocs for the JCL over SLF4J are available at
http://www.slf4j.org/api/overview-summary.html