Panel.describe() method documented but raises NotImplementedError
Panel.describe() docs for v15.2 [here](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Panel.describe.html)

**Environment:**

fedora 20
pandas 15.2
Python 2.7.5
IPython 2.3.0

**To duplicate:**

```
$ pip install --upgrade pandas

In [8]: thresh = pd.Panel({'1': pd.DataFrame(pd.np.arange(2,3)), '2': pd.DataFrame(pd.np.arange(1,2))})

In [9]: thresh
Out[9]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 2 (items) x 1 (major_axis) x 1 (minor_axis)
Items axis: 1 to 2
Major_axis axis: 0 to 0
Minor_axis axis: 0 to 0

In [10]: thresh['1'].describe()
Out[10]: 
        0
count   1
mean    2
std   NaN
min     2
25%     2
50%     2
75%     2
max     2

In [11]: thresh.describe()
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
/usr/src/projects/predict/predict/webapp/dashboard/models.pyc in <module>()
----> 1 thresh.describe()

/usr/local/share/.virtualenvs/predict/lib/python2.7/site-packages/pandas/core/generic.pyc in describe(self, percentile_width, percentiles, include, exclude)
   3748         if self.ndim >= 3:
   3749             msg = "describe is not implemented on on Panel or PanelND objects."
-> 3750             raise NotImplementedError(msg)
   3751 
   3752         if percentile_width is not None and percentiles is not None:

NotImplementedError: describe is not implemented on on Panel or PanelND objects.

In [12]: pd.__version__
Out[12]: '0.15.2'

In [13]: type(thresh)
Out[13]: pandas.core.panel.Panel
```
