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)
I think I am cheating a little bit here... this is the ColumnDataSource:
```
boxplot.source.data
{'cat': ['bronze', 'silver', 'gold'],
 'iqr_cp_listbronze': [2.5, None, None],
 'iqr_cp_listgold': [None, None, 2.5],
 'iqr_cp_listsilver': [None, 2.5, None],
 'iqr_listbronze': [3.0, None, None],
 'iqr_listgold': [None, None, 4.5],
 'iqr_listsilver': [None, 3.0, None],
 'l_cp_listbronze': [1.25, None, None],
 'l_cp_listgold': [None, None, 1.125],
 'l_cp_listsilver': [None, 1.5, None],
 'l_he_listbronze': [0.5, None, None],
 'l_he_listgold': [None, None, 1.75],
 'l_he_listsilver': [None, 1.0, None],
 'lower_listbronze': [-3.0, None, None],
 'lower_listgold': [None, None, -4.75],
 'lower_listsilver': [None, -2.5, None],
 'out_xbronze': ['bronze', 'bronze', 'bronze', 'bronze'],
 'out_xgold': [],
 'out_xsilver': ['silver', 'silver'],
 'out_ybronze': [7.0, 10.0, 8.0, 7.0],
 'out_ygold': [],
 'out_ysilver': [8.0, 8.0],
 'q0bronze': [1.0, None, None],
 'q0gold': [None, None, 0.25],
 'q0silver': [None, 1.0, None],
 'q2bronze': [4.0, None, None],
 'q2gold': [None, None, 4.75],
 'q2silver': [None, 4.0, None],
 'u_cp_listbronze': [2.75, None, None],
 'u_cp_listgold': [None, None, 3.375],
 'u_cp_listsilver': [None, 3.0, None],
 'u_he_listbronze': [2.5, None, None],
 'u_he_listgold': [None, None, 2.75],
 'u_he_listsilver': [None, 2.0, None],
 'upper_listbronze': [6.0, None, None],
 'upper_listgold': [None, None, 8.75],
 'upper_listsilver': [None, 6.5, None],
 'width': [0.8, 0.8, 0.8]}
```

In fact, each cat_"prop" list is filled with the proper data, and all the other ones are None...
So, related with your general comment below, "deleting" one category just involve the NO creation all these related list... so, we one cat I have x list associated with props, for two cats I have 2x and so on...
Maybe a little cheaty... my first idea was to fill the list outside the loop (it is in the commit history), then I thought this idea makes better use of the `_set_and_get` looping strucuture, but maybe it is difficult to understand...