docs: function compareScreenshot(path1, path2, compareOpts) {
      var compareOptions = _.extend({}, options, compareOpts || {});
  
      var deferred = Q.defer();
      // console.log('compareScreenshot - compareOptions:', compareOptions);
      gm.compare(path1, path2, compareOptions, function (err, isEqual, equality /*, raw*/) {
        // console.log('gm compare results:', err, isEqual, equality, raw);
        if (err) { throw err; }
        isEqual ? deferred.resolve(equality) : deferred.reject([path2, ' is not equal to ', path1, ' (tolerance:', compareOptions.tolerance, ', result:', equality, ')'].join(''));
      });
      return deferred.promise;
    }