Factor out the main pipeline from YodaQA_* to pipeline.YodaQA
Use slf4j for logging instead of ad-hoc System.err.println()

This also fixes the situation when we would be locally enabling
some debug AEs in some aggregates, sometimes forget about it, etc.

slf4j seems pretty flexible and mainly has the SimpleLogger!
I still want my logs on stderr and SimpleLogger is easy to use
while I didn't really get the other standard Java logging engines.