VIS: default LinePlot rotation of 0
Closes https://github.com/pydata/pandas/issues/8150

``` python
In [1]: s = pd.Series([1,2,3])

In [2]: s.plot()
Out[2]: <matplotlib.axes._subplots.AxesSubplot at 0x10da02d68>
```

![ex](https://cloud.githubusercontent.com/assets/1312546/4310965/f43fd70c-3eaa-11e4-9e2a-aed38bd73ae3.png)

No release notes since this is just changing it back to how it was for the last release.

It also appeaered in among others histograms (http://pandas-docs.github.io/pandas-docs-travis/visualization.html#histograms), boxplots (http://pandas-docs.github.io/pandas-docs-travis/visualization.html#box-plots). But I suppose this is fixed because they inherited from LinePlot?

Can you also test an irregular time series? (this should be rotated)
 Nope, not fixed. Will do.
 - [x]  ts plot
- [x]  x, y (lineplot)
- [x]  Histograms (sidenote: `by` -> 90º rotation, without by it's 0º)
- [x]  Boxplot (w/ vert=False also)
- [x]  Area
- [x]  KDEplot
- [x] irregular ts (should be rotated)
 Ok, should more or less be working. I had to special case the `x_compat` or irregular timeseries cases since they have a default rot of 30. Probably not the most elegant way. How would I get a png of the rendered docs for you all to see?

I'm also a bit concerned about: http://pandas-docs.github.io/pandas-docs-travis/visualization.html#using-layout-and-targetting-multiple-axes

Have the axes always been messed up on those subplots?
 @TomAugspurger these subplots is largely a new feature in 0.15, so maybe that should be cleaned up before the release. 

For the docs, I think @jreback uses some browser plugin to make a png (see his comment here: https://github.com/jnmclarty/pandas/commit/89d982e089a9b2c0ca4a0ca5c165d514ee47e90a#commitcomment-7776971). Other possibility is to push the html page to a github pages repo (see eg https://github.com/jorisvandenbossche/example-pandas-docs)
 Cool thanks. I'll take the easy (lazy/nearsighted) way and just upload the pngs from my build:

area: 
![area_plot_stacked](https://cloud.githubusercontent.com/assets/1312546/4325472/9b290830-3f65-11e4-9a35-94acea94718f.png)

box:

![box_plot_ex](https://cloud.githubusercontent.com/assets/1312546/4325476/a4b74a60-3f65-11e4-8ac6-d8be1254f3a8.png)

line / x,y:

![df_plot_xy](https://cloud.githubusercontent.com/assets/1312546/4325485/a90c70ae-3f65-11e4-9ef7-175743a84bb8.png)

kde:

![kde_plot](https://cloud.githubusercontent.com/assets/1312546/4325488/afb156a4-3f65-11e4-902a-c52493f849aa.png)

ireg ts:
![ser_plot_suppress_context](https://cloud.githubusercontent.com/assets/1312546/4325509/e00581b8-3f65-11e4-9c08-7491f8d92c9d.png)

hist:

![hist_new](https://cloud.githubusercontent.com/assets/1312546/4325532/10d94338-3f66-11e4-8e8a-c9b892fd0b93.png)
 I'll look into the axes labeling on those plots. Maybe just do it in this PR.
 Mmm, ok, here's what's going on.

![ex](https://cloud.githubusercontent.com/assets/1312546/4326616/3417033e-3f71-11e4-97fa-8b6a82ee28c5.png)

When we do `subplots=True`, `sharex` is set to True, so the top left plot doesn't get a xticklabels (intentional change in https://github.com/pydata/pandas/pull/7457). The others are overlapping because the figure's too small. 

I say we just change it to:

``` python
df.plot(subplots=True, layout=(3, 2), figsize=(6, 6), sharex=False);
```

![ex2](https://cloud.githubusercontent.com/assets/1312546/4326718/2efabc78-3f72-11e4-811e-bb3802729d31.png)

We can make https://github.com/pydata/pandas/issues/8312 into an all pre 0.15 visualization docs tweaking issue and do it there.
 I think this PR is ready if we're handling the other doc issues in a separate PR.

Unless anyone can see a cleaner way of doing this without the `_set_rot` flag I added, which seems really inelegant. We check whether it's an irregular time series or `x_compat` much later, so we need to know whether the default was used or whether it was explicitly  passed in.
 @TomAugspurger I updated #8312 with your comment on the mutliple axes layout issue to make that the issue with the needed visualisation docs tweaking

And for this PR, it seems not that hacky to me
