[SPARK-8452] [SPARKR] expose jobGroup API in SparkR

This pull request adds following methods to SparkR:

```R
setJobGroup()
cancelJobGroup()
clearJobGroup()
```
For each method, the spark context is passed as the first argument. There does not seem to be a good way to test these in R.

cc shivaram and davies

Author: Hossein <hossein@databricks.com>

Closes #6889 from falaki/SPARK-8452 and squashes the following commits:

9ce9f1e [Hossein] Added basic tests to verify methods can be called and won't throw errors
c706af9 [Hossein] Added examples
a2c19af [Hossein] taking spark context as first argument
343ca77 [Hossein] Added setJobGroup, cancelJobGroup and clearJobGroup to SparkR