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)
Well, either is acceptable but the output is different. The `.. note:` way gets you a big highlighted box like here:

http://bokeh.pydata.org/docs/reference.html#bokeh.plotting.multi_line

The `Notes` section gets you a simple Notes section like here:

http://bokeh.pydata.org/docs/reference.html#bokeh.plotting.circle

I guess either is ok but we should be consistent. If you adopt `Notes:` can you go back and change the other versions in the other docstrings? There should not be many.