[MRG] improved Hist.quantiles method

```python
    def quantiles(self, quantiles, strict=False, recompute_integral=False):     
        """                                                                     
        Calculate the quantiles of this histogram                               
                                                                                
        Parameters                                                              
        ----------                                                              
                                                                                
        quantiles : list or int                                                 
            A list of cumulative probabilities or an integer used to determine  
            equally spaced values between 0 and 1 (inclusive).                  
                                                                                
        strict : bool, optional (default=False)                                           
            If True, then return the sorted unique quantiles corresponding      
            exactly to bin edges of this histogram.                             
                                                                                
        recompute_integral : bool, optional (default=False)                     
            If this histogram was filled with SetBinContent instead of Fill,    
            then the integral must be computed before calculating the           
            quantiles.                                                          
                                                                                
        Returns                                                                 
        -------                                                                 
                                                                                
        output : list or numpy array                                            
            If NumPy is importable then an array of the quantiles is returned,  
            otherwise a list is returned.                                       
                                                                                
        """
```