BUG: DataFrame.hist passes keyword arguments to mpl.hist; closes #3296
This is a fix for https://github.com/pydata/pandas/issues/3296 .

Now keyword arguments to `plotting.grouped_hist` are passed to `matplotlib.Axes.hist`. This allows you to create e.g. normalized, cumulative, or log scaled histograms::

```
df = pd.DataFrame(np.random.randn(500, 2), columns=['A', 'B'])
df['C'] = np.random.randint(0, 4, 500)
df.hist(column=['A'], by=df.C, normed=True)
```

PS: the guideline says I should enable Travis-CI on my fork - is there a tutorial how to do that? thx!
