VIS/ENH Hexbin plot

This is just 10 minutes of copy-paste cargo-culting to gauge interest, I haven't tested anything yet.

```python
In [1]: df = pd.DataFrame(np.random.randn(1000, 2))

In [2]: df.plot(kind='hexbin', x=0, y=1)
Out[2]: <matplotlib.axes.AxesSubplot at 0x10eb76e10>
```

![hexbin](https://f.cloud.github.com/assets/1312546/1506888/ef198d6a-4953-11e3-8ecc-891d22def090.png)

It's not terribly difficult to do this on your own, so my feeling wouldn't be hurt at all if people are -1 on this :)

EDIT: oops I branched from my `to_frame` branch.  I'll clean up the commits.