New: stacked barplot

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:

![screen shot 2013-11-20 at 12 53 55 pm](https://f.cloud.github.com/assets/593945/1584994/232676c6-5215-11e3-928c-30f611fb1bbc.png)