[SPARK-17376][SPARKR] Spark version should be available in R

## What changes were proposed in this pull request?

Add sparkR.version() API.

```
> sparkR.version()
[1] "2.1.0-SNAPSHOT"
```

## How was this patch tested?

manual, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14935 from felixcheung/rsparksessionversion.