indexed_chain: allow log to be sent to stdout

[gerbaudo]

This can be useful when running on a batch system; otherwise
the information is written to a log file locally on the worker node,
and then lost. see
http://stackoverflow.com/questions/17523173/python-logging-streamhandler-is-not-sending-logs-to-stdout
and
http://stackoverflow.com/questions/14058453/making-python-loggers-output-all-messages-to-stdout-in-addition-to-log