ENH/CLN: add HistPlot class inheriting MPLPlot

Because ``hist`` and ``boxplot`` are separated from normal ``plot``, there are some inconsistencies with these functions. Looks better to include them to ``MPLPlot`` framework.

Maybe ``scatter`` and ``hist`` can be deprecated in 0.15 if ``MPLPlot`` can offer better ``GroupBy`` plot (plan to do in separate PR).

### Example
This allows to use ``kind='hist`` in ``DataFrame.plot`` and ``Series.plot``. (No changes for ``DataFrame.hist``)
```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

df = pd.DataFrame(np.random.randn(1000, 5))
df.plot(kind='hist', subplots=True)
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/3712412/16aa1896-1513-11e4-863d-e0b6d631c12f.png)

```
df.plot(kind='hist', stacked=True)
```

![figure_2](https://cloud.githubusercontent.com/assets/1696302/3712413/1bded702-1513-11e4-9ff3-3947783d5213.png)

### Remaining Items
- [x] Add a release note in API section detailing this change/enhancement
- [x] Modify doc
- [x] Add tests (both histogram and kde) 
    - [x] Add support for ``rot`` and ``fontsize`` (depending on ``orientation`` kw) (rely on #7844)
    - [x] Add tests for xticklabels and yticklabels
    - [x] Add tests for colors
    - [x] Handling nan
- [x] Check ``stacked=True`` can be supported (``DataFrame.hist`` doesn't support it though..).
    - [x] Add tests for stacking