'sharex' keyword doesn't work for df.plot(kind='hist')
#### Code Sample:

Dataframe:

```
    gre         gpa  prestige
0   380.0   3.61     3.0
1   660.0   3.67     3.0
2   800.0   4.00     1.0
3   640.0   3.19     4.0
4   0520.0  2.93     4.0
```

`df.plot.hist(figsize=(20, 10), layout=(2,3), subplots=True, sharex=False, bins=10);`

Result:
![image](https://cloud.githubusercontent.com/assets/604138/16180645/654894b4-3659-11e6-95ab-880e421fe6a3.png)

Note that scales are all off
#### Expected Output

Independent axis scale for subplots
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: None
numpy: 1.11.0
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: 1.2.2
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.0
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
```
