By-keyword, Inconcistencies between the new df.plot.hist and its df counterpart
Found this when working on 

https://github.com/pydata/pandas/pull/11441

``` python
import pandas as pd
d = {'one' : ['A', 'A', 'B', 'B', 'C'],
     'two' : [4., 3., 2., 2, 1],
     'three' : [10., 8., 3, 5, 7.]}     
df = pd.DataFrame(d)

# this works
df.hist('two', by='one', bins=range(0, 10))

# this does not work (everything in one plot), also no way to specify column
df.plot.hist(by='one', bins=range(0, 10))
```

My idea was to make the df.plot.hist version similar to the df.hist. But the code is much more complex. Would it not be best to point the df.plot.hist to the df.hist version? Instead of having two separate logics for this?

Oh, and the by keyword does not seem to work for df.plot.box, have not found any it worked for. At least the way I expected it to work :)
