Numpy FutureWarning in DataFrame.unstack()
Unstacking with an integer level seems to raise a FutureWarning from numpy.  I'm using pandas version 0.17.0 and numpy version 1.10.1.

```
In[14]: d.unstack('ticker')
Out[14]: 
factor      close          open       
ticker        BMO     TD    BMO     TD
date                                  
2005-08-01  61.10  55.90  61.75  56.60
2005-08-02  62.15  56.77  61.18  55.80
2005-08-03  61.40  55.79  61.50  55.49
2005-08-04  59.09  54.81  60.96  55.15
          ...    ...    ...    ...
2015-10-09  75.51  53.25  76.09  53.70
2015-10-12  75.51  53.25  76.09  53.70
2015-10-13  75.35  52.97  75.51  52.95
2015-10-14  74.41  52.37  75.31  53.06
[2663 rows x 4 columns]

In[15]: d.unstack(1)
C:\Anaconda3\envs\pyq\lib\site-packages\pandas\core\index.py:4281: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  return np.sum(name == np.asarray(self.names)) > 1

Out[15]: 
factor      close          open       
ticker        BMO     TD    BMO     TD
date                                  
2005-08-01  61.10  55.90  61.75  56.60
2005-08-02  62.15  56.77  61.18  55.80
2005-08-03  61.40  55.79  61.50  55.49
2005-08-04  59.09  54.81  60.96  55.15
          ...    ...    ...    ...
2015-10-09  75.51  53.25  76.09  53.70
2015-10-12  75.51  53.25  76.09  53.70
2015-10-13  75.35  52.97  75.51  52.95
2015-10-14  74.41  52.37  75.31  53.06
[2663 rows x 4 columns]
```
