add png visualization in ipython notebook

Add a _repr_png_ method to ghost to give a png preview of plots in the
notebook.

pygal/ghost.py

@@ -102,3 +102,7 @@ class Ghost(object):
         """Render the graph, convert it to png and write it to filename"""
         import cairosvg
         return cairosvg.svg2png(bytestring=self.render(), write_to=filename)
+
+    def _repr_png_(self):
+        """Display png in IPython notebook"""
+        return self.render_to_png()