Panel.clip() doesn't accept (even) scalar values
Let's take the following example:

``` python
d = {'Item1' : pandas.DataFrame(numpy.random.randn(4, 3)),
     'Item2' : pandas.DataFrame(numpy.random.randn(4, 2))}

p = pandas.Panel(d)
p.clip(0,1)
```

If I run the script with pandas 0.14.1 on both python 2.7.8 and 3.4.1 I'm getting the following error:

```
Traceback (most recent call last):
  File "Untitled.py", line 11, in <module>
    p.clip(0,1)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/generic.py", line 2684, in clip
    result = result.clip_lower(lower)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/generic.py", line 2722, in clip_lower
    return self.where((self >= threshold) | isnull(self), threshold)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 934, in f
    self._constructor.__name__)
ValueError: Simple arithmetic with Panel can only be done with scalar values
```

Digging a little bit deeper showed that the following line (the | operator to be precise) raises the error:
https://github.com/pydata/pandas/blob/1d65bc89d64c71f8d36f3ca92dd57db2efad7fdb/pandas/core/generic.py#L2796
