ENH: Allow different number of rows & columns in histogram plot
It didn't seem necessary that a grid of histogram plots should be square.  For example, if you have 6 columns to plot, a 2x3 grid makes more sense than 3x3 with an empty row.  Likewise with 12 it should give you 3x4, and so on.

I added a simple way to allow this. The number of columns will not exceed the number of rows by more than 1 (e.g., you will not get a 2x5 grid if you have 10 columns).

For a quick test:

``` python
df = pandas.DataFrame(np.random.randn(200, 6))
df.hist()
```

Note: it might be desirable to allow more rows than columns for plots in portrait mode.  That could easily be a keyword option.
