Refactor: Legends & fill

[has2k1]

Legend information is created in the assign functions called by components.assign_visual_mapping and it is attached to the ggplot object to create
the legend dictionary.

{aesthetic: {visual_value: legend_key}}

# where
aesthetic := 'color' | 'linestyle' | 'marker' | 'size'
visual_value := color | linestyle | marker-character | size-value
legend_key := quantile

The refactoring would have the legend modified to

{aesthetic: (column_name, {visual_value: legend_key})}

# where
aesthetic := 'color' | 'fill' | 'linetype' | 'shape' | 'size' | 'alpha'
column_name := 'cat', 'cat2', 'factor(cyl)', ...
visual_value := color | fill-color | linetype | shape-character | size-value | alpha-value
legend_key := quantile

And no ggplot object would be passed, instead assign_visual_mapping would return the legend, so each layer would be able to control the legend, #211. The changes also include doing away with the matplotlib API names, this has cascading effects, https://github.com/yhat/ggplot/blob/master/ggplot/geoms/geom.py#L74. Changes would also be made to handle the fill mapping. Keeping the column name would allow for smarter legends later on, #89.

This is issue is in someway a blocker for #266.