Plotting Timedelta on y-axis
#### Code Sample

```python
import pandas as pd
from pandas.compat import StringIO
import matplotlib.pyplot as plt


dat = """c1,c2,c3
1000,2000,1500
9000,8000,1600"""

df = pd.read_csv(StringIO(dat))

print(df)

df.plot.bar()
plt.show()

df = df.apply(lambda x: pd.to_timedelta(x, unit='ms'))

print(df)

df.plot.bar()
plt.show()

```
#### Problem description

`plot.bar()` works with numerical values (`int`, `float`).

```
     c1    c2    c3
0  1000  2000  1500
1  9000  8000  1600
```

![capture d ecran 2017-07-15 a 20 20 30](https://user-images.githubusercontent.com/109167/28241986-6b0fb8e8-69a0-11e7-88d0-2c32de28af90.png)

not with `Timedelta`

```
        c1       c2              c3
0 00:00:01 00:00:02 00:00:01.500000
1 00:00:09 00:00:08 00:00:01.600000
```

```
    df.plot.bar()
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2659, in bar
    return self(kind='bar', x=x, y=y, **kwds)
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2620, in __call__
    sort_columns=sort_columns, **kwds)
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 1857, in plot_frame
    **kwds)
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 1682, in _plot
    plot_obj.generate()
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 236, in generate
    self._compute_plot_data()
  File "/Users/scls/anaconda/lib/python3.6/site-packages/pandas/plotting/_core.py", line 345, in _compute_plot_data
    'plot'.format(numeric_data.__class__.__name__))
TypeError: Empty 'DataFrame': no numeric data to plot
```

#### Expected Output

Timedelta on y axis should be displayed

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

<details>
/Users/scls/anaconda/lib/python3.6/site-packages/xarray/core/formatting.py:16: FutureWarning: The pandas.tslib module is deprecated and will be removed in a future version.
  from pandas.tslib import OutOfBoundsDatetime

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.20.1
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: 0.9.5
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.4.0
</details>
