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
