Added functionality for Bar charts and HorizontalBar charts to put the value label on top of the bar, bottom of the bar or the default, middle of the bar.

Example use:
chart = pygal.Bar(position_values='top')
chart.add('Series 1', [1, 3, 43, 2.5, 34, 1.4])
chart.render()

pygal/css/graph.css

@@ -22,7 +22,6 @@
   text-anchor: middle;
 }
 
-
 {{ id }}.centered {
   text-anchor: {{ style.align }};
 }