[SPARK-18958][SPARKR] R API toJSON on DataFrame

## What changes were proposed in this pull request?

It would make it easier to integrate with other component expecting row-based JSON format.
This replaces the non-public toJSON RDD API.

## How was this patch tested?

manual, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16368 from felixcheung/rJSON.