Panel.clip() does not work 
Hello,
I tried to use the clip function on a basic Panel but it seems that the axis management is not working as expected. I believe this is an update to issue https://github.com/pydata/pandas/issues/8344

```
import numpy as np
import pandas as pd

data = np.random.randn(3, 4, 5)
panel = pd.Panel(data)
panel.clip(0, 1)
```

If I run the script with pandas 0.16.2 on python 2.7.10 and 3.4.3, I'm getting the following error:

```
.../pandas/core/generic.pyc in clip(self, lower, upper, out, axis)
   3052         result = self
   3053         if lower is not None:
-> 3054             result = result.clip_lower(lower, axis)
   3055         if upper is not None:
   3056             result = result.clip_upper(upper, axis)

.../pandas/core/generic.pyc in clip_lower(self, threshold, axis)
   3103             raise ValueError("Cannot use an NA value as a clip threshold")
   3104 
-> 3105         subset = self.ge(threshold, axis=axis) | isnull(self)
   3106         return self.where(subset, threshold, axis=axis)
   3107 

TypeError: f() got an unexpected keyword argument 'axis'
```

**Thanks a lot for all your efforts!**
