make R.isEmpty equivalent to _.isEmpty

Closes #538

[Underscore][1]:

```javascript
// Is a given array, string, or object empty?
// An "empty" object has no enumerable own-properties.
_.isEmpty = function(obj) {
  if (obj == null) return true;
  if (_.isArray(obj) || _.isString(obj) || _.isArguments(obj)) return obj.length === 0;
  for (var key in obj) if (_.has(obj, key)) return false;
  return true;
};
```


[1]: https://github.com/jashkenas/underscore/blob/1.7.0/underscore.js#L1068-L1075