[SPARK-20360][PYTHON] reprs for interpreters

## What changes were proposed in this pull request?

Establishes a very minimal `_repr_html_` for PySpark's `SparkContext`.

## How was this patch tested?

nteract:

![screen shot 2017-04-17 at 3 41 29 pm](https://cloud.githubusercontent.com/assets/836375/25107701/d57090ba-2385-11e7-8147-74bc2c50a41b.png)

Jupyter:

![screen shot 2017-04-17 at 3 53 19 pm](https://cloud.githubusercontent.com/assets/836375/25107725/05bf1fe8-2386-11e7-93e1-07a20c917dde.png)

Hydrogen:

![screen shot 2017-04-17 at 3 49 55 pm](https://cloud.githubusercontent.com/assets/836375/25107664/a75e1ddc-2385-11e7-8477-258661833007.png)

Author: Kyle Kelley <rgbkrk@gmail.com>

Closes #17662 from rgbkrk/repr.