add png visualization in ipython notebook

Add a _repr_png_ method to ghost to give a png preview of plots in the
notebook.

For example, entering the following into a cell will display a graph:

```python
import pygal                                                       # First import pygal
bar_chart = pygal.Bar()                                            # Then create a bar graph object
bar_chart.add('Fibonacci', [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55])  # Add some values
bar_chart
```