[MRG] New general evaluate() function

Evaluate ROOT histograms, functions, graphs, and splines over NumPy arrays.

```python
>>> from root_numpy import evaluate                                         
>>> from ROOT import TF1, TF2                                               
>>> func = TF1("f1", "x*x")                                                 
>>> evaluate(func, [1, 2, 3, 4])                                            
array([  1.,   4.,   9.,  16.])                                             
>>> func = TF2("f2", "x*y")                                                 
>>> evaluate(func, [[1, 1], [1, 2], [3, 1]])                                
array([ 1.,  2.,  3.])
```