BUG: barplot with NaNs
Closes https://github.com/pydata/pandas/issues/8175

This makes barplot more consistent with how NaN's are handled in AreaPlots and PiePlots.

I'll add this argument to `df.plot(..., fillna=None)` and `series.plot(..., fillna=None)`. The default is `None`, which means follow these defaults:
- Line Plot: Don't fill anything, leaves gaps in the line at the missing values
- (Stacked) Bar Plot: Fill with zeros before plotting (This PR fixes the bug on stacked bar plots not drawing past a NaN).
- Hist: `df.plot(kind='hist')` <==> `df.dropna().plot(kind='hist')` (column-wise I think)
- Boxplot: `df.plot(kind='box')` <==> `df.dropna().plot(kind='box')` (column-wise I think)
- Areaplot `df.plot(kind='area')` <==> `df.fillna(0).plot(kind='area')`
- Hexbin: `df.plot(kind='hexbin')` <==> `df.dropna().plot(kind='area')`
- Pie: `df.plot(kind='pie')` <==> `df.fillna(0).plot(kind='pie')`

All look good? The only end-user change from this PR is barplot not bugging out with NaNs.
The reason that I don't see a need for a kwarg is that I don't really think there's anything to turn off here. I think that for all of these, the default is the _only_ sensible thing to do. Any counterexamples where our defaults do the wrong thing?
 Complement:
- Line Plot(unstacked): Don't fill anything, leaves gaps in the line at the missing values
- Line Plot(stacked): `df.fillna(0).plot(stacked=True)`
- Kde: `df.dropna().plot(kind='kde')`
- Scatter: `df.dropna().plot(kind='scatter')`
 +1 for _not_ a new keyword, let's just implement sane default behaviour (which is already the case I think apart from the bug addressed iin this PR), and let people use `fillna` if they want something else.

One remark, the default for `pie` is fill with 0, would it not more logical to drop?
 Re the default for pie, I think you're correct.

``` python
In [11]: series = Series(3 * np.random.rand(8),
    index=['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'], name='series')
In [15]: series.iloc[:4] = np.nan

In [16]: series.plot(kind='pie')
Out[16]: <matplotlib.axes._subplots.AxesSubplot at 0x11183e128>
```

![nana](https://cloud.githubusercontent.com/assets/1312546/4175194/c8b246d6-35c4-11e4-9513-b1539f956d72.png)

A bit hard to see, but all the NaNs are piled on top of each other (middle right). New issue?
