[ENH - plot] Would you welcome a DataFrame.plot.gantt() contribution?
### Example
Given the following dataframe:

```python
data = {'Class': ['bonds', 'bonds', 'bonds', 'bonds', 'futures', 'options', 
                  'futures', 'etf', 'equities', 'equities', 'equities', 'equities'], 
        'Start': [0.006568, 0.006663, 0.008089, 0.008185, 0.033096, 0.034997, 
                  0.039969, 0.059823, 0.068865, 0.068962, 0.083492, 0.083588], 
        'Finish': [0.00662, 0.006695, 0.008141, 0.008217, 0.035016, 0.037077, 
                   0.042057, 0.061639, 0.068917, 0.068994, 0.083544, 0.08362], 
        'Position': ['long', 'long', 'short', 'long', 'short', 'short', 'short', 
                     'long', 'long', 'long', 'long', 'long']}

df = pd.DataFrame(data)
```
<img width="275" alt="screen shot 2018-04-20 at 13 11 05" src="https://user-images.githubusercontent.com/3731173/39050546-ba079ab4-449d-11e8-9a40-c3e731df7bff.png">

it would plot:

```python
df.plot.gantt(task='Class', start='Start', end='Finish', by='Position')
```

<img width="446" alt="screen shot 2018-04-20 at 13 21 53" src="https://user-images.githubusercontent.com/3731173/39050575-d504c2f6-449d-11e8-8dc4-68f58a37cc8b.png">

### Applicability
Gantt charts are used to plot:
* schedules (i.e. task over-time, their completion status etc...) 
* usage cases, e.g. how long has a certain feature been used compared to another
* in finance, to get an overview of a portfolio of trades in order to understand and monitor investment strategies (do they work, what went wrong etc...)

### Key question
**Would you welcome a PR under pandas.DataFrame.plot()?**

The implementation would either wrap around [matplotlib.pyplot.eventplot](https://matplotlib.org/api/_as_gen/matplotlib.pyplot.eventplot.html#matplotlib.pyplot.eventplot) or [matplotlib.pyplot.broken_barh](https://matplotlib.org/api/_as_gen/matplotlib.pyplot.broken_barh.html#matplotlib.pyplot.broken_barh)