DOC: figures not sharp but little bit blurry
I noticed in the docs that the figures (in eg http://pandas.pydata.org/pandas-docs/dev/visualization.html) are not really sharp as could be, but somewhat blurry.

So I wondered what the cause of this is, and it seems that because the `@savefig` pseudo-decorator is (usually) set with a `width=6in` the figure becomes a little bit enlarged/shrunken, with the result that the figure is not sharp anymore.

So I tried what it would give if you remove this width. I tried some things (only in visualization.rst and rplot.rst):
- Current situation:  
  http://jorisvandenbossche.github.io/example-pandas-docs/html-plot-current/visualization.html
- `width=6in` removed, so all figures keeping their own size (and becomen, in general a little bit smaller):  
  http://jorisvandenbossche.github.io/example-pandas-docs/html-plot-withoutwidth/visualization.html
- `width=6in` removed, so all figures keeping their own size but set larger in the mpl_stylesheet (from `(6,4)` to `(8,6)`):  
  http://jorisvandenbossche.github.io/example-pandas-docs/html-plot-withoutwidth-larger/visualization.html

I think the second one, with removing the width, is better than the current situation (only df.hist does not seem to follow the default size and is too large with removing the width parameter).
What do you guys think?

@y-p I think you are the last one who touched this to uniform the width of all figures (to 6 inch)
