DOC: Mention date and time formatting available throughout MPL
The visualization documentation only mentions plotting using pandas' interface. This patch adds another section mentions that it is also possible to plot directly with MPL and that date and time formatters are automatically available through all plots in MPL. An example shows matplotlib's errorbar function where points and errors on the x axis are respectively timestamps and offsets.

the error plotting is going to be supported by #5638 
 Ok, I didn't know. I'll prepare another example: is fill_between (planned to be) supported by pandas?
 no....that would be fine
 For your convenience, this is the example plot, now using MPL's stackplot.

![stackplot](https://f.cloud.github.com/assets/1579083/2477667/bfbe77a6-b073-11e3-9a83-e0d03ab4515e.png)
 @jorisvandenbossche look ok ?
 @agijsberts I think this is certainly a good addition to the docs. 
Although I was thinking, the fact that sometimes you need to plot directly with matplotlib, and the date formatting are to somewhat seperate facts that both can be under this title. But maybe seperate them a little bit? (eg just in two paragraphs)?
 Regarding stackplot example, I've prepared PR #6656. I've used `fill_between` to outplot it.
 Bad news for me, good news for pandas ;)

I updated the patch with a simple finance related example.

![bollinger](https://f.cloud.github.com/assets/1579083/2492463/3269552a-b22f-11e3-8386-b890752a3452.png)

Hopefully this will do.
 @agijsberts can you squash this down to a single commit...otherwise looks ok

@jorisvandenbossche @TomAugspurger 
 @jreback Sorry about that, still new to git. Should be ok now; if not, please let me know.
 OK!
