[MRG] hist: new rebinned method for all rebinning cases and for all dimensions


Flatten a histogram by rebinning with the quantiles, for example:

```python
>>> from rootpy.plotting import Hist
>>> a = Hist(500, -5, 30)
>>> a.FillRandom('landau', 10000000)
>>> a.Draw()
>>> b = a.rebinned(a.quantiles(10, strict=True))
>>> b.SetMinimum(0)
>>> b.Draw()
```

```python
    def rebinned(self, bins, axis=0):                                           
        """                                                                     
        Return a new rebinned histogram                                         
                                                                                
        Parameters                                                              
        ----------                                                              
                                                                                
        bins : int, tuple, or iterable                                          
            If ``bins`` is an int, then return a histogram that is rebinned by  
            grouping N=``bins`` bins together along the axis ``axis``.          
            If ``bins`` is a tuple, then it must contain the same number of     
            elements as there are dimensions of this histogram and each element 
            will be used to rebin along the associated axis.                    
            If ``bins`` is another iterable, then it will define the bin        
            edges along the axis ``axis`` in the new rebinned histogram.        
                                                                                
        axis : int, optional (default=0)                                        
            The axis to rebin along.                                            
                                                                                
        Returns                                                                 
        -------                                                                 
                                                                                
        The rebinned histogram                                                  
                                                                                
        """
```