add R.mapEntry

This simple function is useful when working with objects in point-free style. Here's an example from actual code:

```javascript
// toQuestionnaire :: Object -> {type,send:{questions:[{question}]}}
var toQuestionnaire = R.pipe(
  R.prop('credentials'),                // :: {questions:[{question}]}
  R.prop('questions'),                  // :: [{question}]
  R.last,                               // :: {question}
  R.pick(['question']),                 // :: {question}
  R.of,                                 // :: [{question}]
  R.mapEntry('questions'),              // :: {questions:[{question}]}
  R.mapEntry('send'),                   // :: {send:{questions:[{question}]}}
  R.mixin(R.mapEntry('type', 'questions'))
);
```

The term "map entry" is used in Clojure. I agree that it's not a perfect fit here. I'm open to suggestions. :)