Problem getting set difference OR isnotin
I'm using `isin` and want `isnotin`, so I figured this would give it to me:

```
inside = df[df['Col'].isin(good_vals)]
not_in = df - inside
```

I'm getting the following error:

```
  File "sensor.py", line 572, in wrapped
    not_in = df - inside
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/pandas/core/frame.py", line 104, in f
    return self._combine_frame(other, func, fill_value)
  File "/home/mharrison/work/pandas/env/lib/python2.6/site-packages/pandas/core/frame.py", line 1862, in _combine_frame
    result = func(this_vals, other_vals)
TypeError: unsupported operand type(s) for -: 'str' and 'float'
```
