Add optional logback dependency to kbp-events2014
Add optional logback dependency to kbp-events2014

This allows executables in this module to have logging

BBN-E/tac-kbp-eal,rgabbard-bbn/kbp-2014-event-arguments,BBN-E/tac-kbp-eal,rgabbard-bbn/kbp-2014-event-arguments,isi-nlp/tac-kbp-eal,isi-nlp/tac-kbp-eal