using DataFrame.interpolate with inplace=True causes error
I got the error below. Looking at the code, it looks like there's a return statement missing above - the code should never get to this point, or should return the DataFrame given as argument, whenever inplace=True.

```
/Library/Python/2.7/site-packages/pandas/core/generic.pyc in interpolate(self, method, axis, limit, inplace, downcast, **kwargs)
   2304         if axis == 1:
   2305             res = res.T
-> 2306         return res
   2307 
   2308     #----------------------------------------------------------------------

UnboundLocalError: local variable 'res' referenced before assignment
```
