New: stacked barplot

[jonathanrocher]

This is bringing back to life a PR by Travis Vaught to add a new type of plot in Chaco: a stacked barplot, leading plots of this type:

[pberkes]

It would be great to keep the documentation updated:

1. Update the script in https://github.com/enthought/chaco/blob/master/examples/user_guide/plot_types/create_plot_snapshots.py to create an example of the plot
2. Update the documentation page at https://github.com/enthought/chaco/blob/master/docs/source/user_manual/plot_types.rst , documenting the plot-specific attributes

[corranwebster]

And at least a "smoke test"-style unit test!