DataFrame.boxplot() NaN handling
In case of NaN, `DataFrame.boxplot()` produces unexpected plot (see below df2, column A boxplot).
It is matplotlib related (using version 1.1.0).

``` python
In [248]: df = pandas.DataFrame(np.random.randn(1000,2), columns=['A', 'B'])

In [249]: df.describe()
Out[249]:
                 A            B
count  1000.000000  1000.000000
mean      0.015119    -0.000871
std       1.010431     1.028899
min      -2.751259    -2.959695
25%      -0.707412    -0.684895
50%       0.025767    -0.000795
75%       0.724173     0.757402
max       3.885804     2.821532

In [250]: df2 = pandas.concat([df, df], ignore_index=True)

In [251]: df2['A'][1000::] = np.NaN

In [252]: df2.describe()
Out[252]:
                 A            B
count  1000.000000  2000.000000
mean      0.015119    -0.000871
std       1.010431     1.028641
min      -2.751259    -2.959695
25%      -0.707412    -0.684895
50%       0.025767    -0.000795
75%       0.724173     0.757402
max       3.885804     2.821532

In [253]: df.boxplot();figure();df2.boxplot();
```

[screenshot](https://lh4.googleusercontent.com/-7EplwgT9ltk/T-MUlkumNxI/AAAAAAAAANg/lQPskQy56XQ/s800/issue1506_screenshot1.png)
