small fix: add __str__
small fix: add __str__

ehealthafrica-ci/rapidsms,catalpainternational/rapidsms,ken-muturi/rapidsms,caktus/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,ken-muturi/rapidsms,catalpainternational/rapidsms,unicefuganda/edtrac,ken-muturi/rapidsms,ehealthafrica-ci/rapidsms,peterayeni/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms,eHealthAfrica/rapidsms,unicefuganda/edtrac,unicefuganda/edtrac,dimagi/rapidsms,catalpainternational/rapidsms,catalpainternational/rapidsms,dimagi/rapidsms,lsgunth/rapidsms,eHealthAfrica/rapidsms,caktus/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms,eHealthAfrica/rapidsms,peterayeni/rapidsms,caktus/rapidsms