Fix boxplot

Depends on #929.
Some bug fixes over the boxplot chart generated at the scipy sprint. Now it is using the categorical axis in a proper way (instead of an hybrid thing we had before at the time of drawing the segments... it was used some "kind" of linear x values instead of the categories in the categorical axis). Also changed quad for rect, deleted unused things and improve the styling a little bit... now look in this way:

![boxplot](https://cloud.githubusercontent.com/assets/1640669/3768303/11d99bb8-18d6-11e4-8275-62fa411db432.png)

(forget the "cutted" label... it is another issue)
Oh, I think we need to take a step back. A couple of notes: 
* every column in a given column data source should have the same number of elements (we don't enforce this but it is an assumption)
* Using `None` like that seems undesirable
I think something about our examples gives off the notion that creating multiple data sources is bad, but that is not the case. In this case, there should really be many different data sources, basically one for every single renderer. Another benefit of this that now we don't have to try and encode information in long string column names. Each data source an have simple column names. Here is a rough sketch for what data sources a plot with two categories should probably have:
* category FOO
 - quad renderer and data source  (data for 2 quads)
 - segment renderer and data source (data for 4 segments)
 - marker renderer and data source (data for N outliers)
* category BAR
 - quad renderer and data source  (data for 2 quads)
 - segment renderer and data source (data for 4 segments)
 - marker renderer and data source (data for M outliers)