Adds custom plot formatting for TimedeltaIndex.
Instead of hacking the PeriodIndex plotting code to work for TimedeltaIndex, this just creates a simple formatter unique to TimedeltaIndex. It does fine for regular intervals up to a day, then the labels and run into eachothter. If this happens, you can use plt.autofmt_xdate() to fix it.

 - [x] closes #8711
 - [ ] tests  passed, none added
 - [x] passes ``git diff upstream/master | flake8 --diff``


```python
d.t_sensor_gyro_0__f_x.plot()
plt.grid()
```
![image](https://cloud.githubusercontent.com/assets/473772/21677741/76393d44-d309-11e6-90aa-b94fd4e2e989.png)


```python
index =  pd.timedelta_range('1 day 2 hr 30 min 10 s', periods=10, freq='1 d')
s = pd.Series(np.random.randn(len(index)), index)
s.plot()
plt.gcf().autofmt_xdate()
plt.grid()
```

![image](https://cloud.githubusercontent.com/assets/473772/21677435/24779c86-d308-11e6-9350-2251a8eca334.png)


```python
index =  pd.timedelta_range('1 day 2 hr 30 min 10 s', periods=10, freq='1 ns')
s = pd.Series(np.random.randn(len(index)), index)
s.plot()
plt.gcf().autofmt_xdate()
plt.grid()
```

![image](https://cloud.githubusercontent.com/assets/473772/21677429/1bd28794-d308-11e6-9ed6-72fa831a60f9.png)

```python
index =  pd.timedelta_range('0s', periods=10, freq='100000 ns')
s = pd.Series(np.random.randn(len(index)), index)
s.plot()
plt.gcf().autofmt_xdate()
plt.grid()
plt.xlabel('time')
plt.ylabel('y')
```

![image](https://cloud.githubusercontent.com/assets/473772/21677938/7ef8392a-d30a-11e6-8c5c-f77cc0f49a7c.png)

