Titles of subplots are shown when kind='bar' but not when kind='line'
The problem is described in this stackoverflow post: [stackoverflow](http://stackoverflow.com/questions/42812105/titles-of-subplots-are-shown-when-kind-bar-but-not-when-kind-line).

#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import itertools

df = pd.DataFrame(np.random.randn(20, 10), columns=list('ABCDEFGHIJ'))

ax = df.plot(kind='bar', subplots=True, layout=(4, 3), sharex=True, sharey=False)

plt.show()
```
#### Problem description
The code above gives a plot where the column headers are used as titles of the subplots. However, when `kind='bar'` is changed to `kind='line'`, there are not titles at the individual subplots.

#### Expected Output
To make it consistent, titles should always be displayed, independent of the value of the `kind` flag. If one uses the latest version of [plotting.py](https://github.com/pandas-dev/pandas/blob/998c801f76256990b98d3f0d2ad885ae27c955a1/pandas/tools/plotting.py) and passes `title=list(df.columns.values)`, it works fine.
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 3.16.0-70-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.1
nose: 1.3.4
pip: 9.0.1
setuptools: 25.0.1
Cython: 0.22
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 5.0.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.36.0
pandas_datareader: None

</details>
