[MRG] hist: add poisson_errors method

```python
from rootpy.plotting import Hist, Canvas
from rootpy.interactive import wait
hist = Hist(20, -3, 3)
hist.FillRandom('gaus')
graph = hist.poisson_errors()
c = Canvas()
c.SetLogy()
hist.Draw()
d = Canvas()
d.SetLogy()
graph.Draw('AP')
graph.xaxis.SetLimits(-3, 3)
d.RedrawAxis()
wait()
```
![4dc4e6228a75495082dc1974dead7db7](https://f.cloud.github.com/assets/202816/1563741/46ec866a-5065-11e3-882e-b2e3fa82b09c.png)
![50cec56108c3405882d95e4ea2dfc31a](https://f.cloud.github.com/assets/202816/1563743/4a80db14-5065-11e3-9400-cc2d2a624f78.png)