Improve plotting Timedelta on y-axis (Timedelta string representation)
#### Code Sample, a copy-pastable example if possible

Following https://github.com/pandas-dev/pandas/issues/9154#issuecomment-329814404 and https://github.com/pandas-dev/pandas/issues/16953#issuecomment-329905621

```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()
```

Displays

```
        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
```

![figure_1-1](https://user-images.githubusercontent.com/109167/30503515-d3641614-9a6a-11e7-85d4-484e498c0cac.png)

#### Problem description

Y-axis should display Timedelta string representation (not integer values)

#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: 328c7e179b72e257e27adf92a06718fd5a40473f
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.21.0.dev+464.g328c7e179
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.26
numpy: 1.13.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
