Added _.propertyOf method

_.propertyOf method as discussed in https://github.com/jashkenas/underscore/issues/1916

Resolves #1916
It's a needless inconsistency and one that `_.property` doesn't have. You could just as easily have the function be:
```js
    function propertyOf(obj) {
      return function(key) {
        return obj == null ? void 0 : obj[key];
      };
    }
```