destroys (retrieve and delete) all entities matching criteria (criteria is optional)
function(criteria, cb) {
        if (_.isFunction(criteria)) { cb = criteria; criteria = undefined; }
        this.findBy(criteria, function(err, entities) {
          if (err) { return cb(err); }
          var deleteEntity = function(entity, callback) {
            entity.delete(callback);
          };
          async.each(entities, deleteEntity, function(err) {
            cb(err);
          });
        });
      }