[MRG] add fill_graph

```python
>>> from root_numpy import fill_graph
>>> from ROOT import TGraph
>>> from numpy import array
>>> 
>>> a = array([[1, 1], [2, 4], [3, 9], [4, 16]])
>>> g = TGraph()
>>> fill_graph(g, a)
>>> g.Draw()
```
![c1](https://f.cloud.github.com/assets/202816/1788895/a38b301a-6942-11e3-851d-d741e41e18fc.png)