scatter_matrix doesn't support passing histogram keyword args
There is no way to pass keyword arguments down to the histogram call in `pandas.tools.plotting.scatter_matrix`.  I understand that it's sitting on top of a few interfaces and so intelligently deciding which keyword arguments go where is complex.  

A simple solution would be to add a `hist_kwargs={}` keyword argument to which users could pass values like `{'bins': 100}` and then use this in the `hist` call like so

```
ax.hist(values, **hist_kwargs)
```
