Series.describe() fails for empty and None series.

Running `describe()` raises and exception for `Series([])` and `Series([None])`.  See the following examples:


    In [416]: Series([]).describe()
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-416-162185f07510> in <module>()
    ----> 1 Series([]).describe()

    /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas-0.8.0rc2-py2.7-macosx-10.4-x86_64.egg/pandas/core/series.pyc in describe(self, percentile_width)
      1363                      'max']
      1364 
    -> 1365             data = [self.count(), self.mean(), self.std(), self.min(),
      1366                     self.quantile(lb), self.median(), self.quantile(ub),
      1367                     self.max()]

    /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas-0.8.0rc2-py2.7-macosx-10.4-x86_64.egg/pandas/core/series.pyc in min(self, axis, out, skipna, level)
      1067         if level is not None:
      1068             return self._agg_by_level('min', level=level, skipna=skipna)
    -> 1069         return nanops.nanmin(self.values, skipna=skipna)
      1070 
      1071     @Substitution(name='maximum', shortname='max',

    /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas-0.8.0rc2-py2.7-macosx-10.4-x86_64.egg/pandas/core/nanops.pyc in f(values, axis, skipna, **kwds)
        41                 result = alt(values, axis=axis, skipna=skipna, **kwds)
        42         except Exception:
    ---> 43             result = alt(values, axis=axis, skipna=skipna, **kwds)
        44 
        45         return result

    /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas-0.8.0rc2-py2.7-macosx-10.4-x86_64.egg/pandas/core/nanops.pyc in _nanmin(values, axis, skipna)
        152             result = __builtin__.min(values)
        153     else:
    --> 154         result = values.min(axis)
        155 
        156     return _maybe_null_out(result, axis, mask)

    ValueError: zero-size array to minimum.reduce without identity

    In [388]: s = pandas.Series([None])

    In [389]: s.describe()
    ---------------------------------------------------------------------------
    IndexError                                Traceback (most recent call last)
    <ipython-input-389-97bd840d09f5> in <module>()
    ----> 1 s.describe()

    /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas-0.8.0rc2-py2.7-macosx-10.4-x86_64.egg/pandas/core/series.pyc in describe(self, percentile_width)
      1334 
      1335             objcounts = Counter(self.dropna().values)
    -> 1336             top, freq = objcounts.most_common(1)[0]
      1337             data = [self.count(), len(objcounts), top, freq]
      1338 

    IndexError: list index out of range

    In [395]: s = pandas.Series([np.nan, np.nan])

    In [396]: s.describe()
    Out[396]: 
    count     0
    mean    NaN
    std     NaN
    min     NaN
    25%     NaN
    50%     NaN
    75%     NaN
    max     NaN

I have included `Series([np.nan])` because its output is affected by the pull request.


After the pull request the output is as follows:

    In [3]: Series([]).describe()
    Out[3]: count    0

    In [4]: Series([None]).describe()
    Out[4]: 
    count     0
    unique    0

    In [5]: Series([np.nan]).describe()
    Out[5]: count    0

The pull request fixes the `ValueError` from the call to `values.min(axis)` and the `IndexError` from accessing `most_common(1)[0]` by only returning a small amount of count data when the Series object is empty or contains all NaN or None values.  Is this the right approach?  It changes the output for Series objects with all NaN values.  

Test coverage was written but I couldn't figure out how to run the tests.  Any hints?

Regards,
Todd