sl4j integration

[paulhoule]

Well, it happened.

I was developing something to build elasticsearch indexes that uses SLF4J and found that it gave error messages because SLF4J was set up.

It is possible to set things up so that there is no commons logging dependency but that a stub is put in that routes commons logging through SLF4J, this is documented for use with Spring.

The main PITA involved is that users of the system will need to systematically suppress commons-logging from coming in a dependency otherwise strange things that can suck huge amounts of time can happen.