added a callback parameter, so the initiator will get informed when an image retrieval has succeeded
added a callback parameter, so the initiator will get informed when an image retrieval has succeeded

ismaelgaudioso/Coherence,furbrain/Coherence,opendreambox/python-coherence,furbrain/Coherence,opendreambox/python-coherence,coherence-project/Coherence,sreichholf/python-coherence,unintended/Cohen,unintended/Cohen,coherence-project/Coherence,sreichholf/python-coherence,ismaelgaudioso/Coherence