DataFrame.plot.box ignores by argument
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({'a': ['one', 'one', 'two', 'one', 'two', 'two'], 
                   'b': [1, 2, -1, 2, -2, -3]})
df.boxplot(by='a')
# two boxplots, one for 'one' and the other for 'two'
# this is an expected behaviour
```
![boxplot-ok](https://cloud.githubusercontent.com/assets/2717321/21741924/dfb41efa-d4f4-11e6-991e-65a4009b83c2.png)

```python
df.plot(kind='box', by='a')
# one boxplot, grouping ignored
```
![boxplot-kind](https://cloud.githubusercontent.com/assets/2717321/21741928/e94131a6-d4f4-11e6-9e39-645c033483b1.png)

```python
df.plot.box(by='a')
# one boxplot, grouping ignored
```
![boxplot-plot-box](https://cloud.githubusercontent.com/assets/2717321/21741933/0730a444-d4f5-11e6-94a2-157963f12a13.png)

#### Problem description

It seems that `by` argument is silently ignored and no grouping occurs. Probably, it is related to #9274.

#### Expected Output

I expect that three figures will be the same as the first one.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0+311.gb895968
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.1
numpy: 1.11.2
scipy: None
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_datareader: None
</details>