ENH : add overwrite kwarg to register_handler
ENH : add overwrite kwarg to register_handler

Allows to forcibly re-set what is in the registry

NSLS-II/filestore,ericdill/fileStore,danielballan/filestore,stuwilkins/filestore,ericdill/fileStore,tacaswell/filestore,ericdill/databroker,danielballan/filestore,stuwilkins/filestore,ericdill/databroker