hist: add poisson_errors method

```python
>>> from rootpy.plotting import Hist
>>> a = Hist(100, -3, 3)
>>> a.FillRandom('gaus')
>>> a.Draw()
>>> b = a.poisson_errors()
>>> b.Draw('same')
>>> b.Draw('same P')
>>> from rootpy.plotting import Canvas
>>> c = Canvas()
>>> a.Draw()
>>> d = Canvas()
>>> b.Draw('AP')
```