code: public Signature getRelatedSignature() throws HelloSignException {
          String id = getRelatedSignatureId();
          if (id == null) {
              return null;
          }
          for (Signature sig : ((SignatureRequest) resource).getSignatures()) {
              if (id.equals(sig.getId())) {
                  return sig;
              }
          }
          return null;
      }
docs: Returns the associated Signature object with this event, if the event is
  associated with a Signature Request.
  
  @return Signature
  @throws HelloSignException thrown if there is a problem parsing the
  backing JSONObject.