DataFrame.describe fails for dtype=datetime64
```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/data/haiti/<ipython-input-165-efa692752119> in <module>()
----> 1 data.describe()

/home/wesm/code/pandas/pandas/core/frame.pyc in describe(self, percentile_width)
   3772         for column in numdata.columns:
   3773             series = self[column]
-> 3774             destat.append([series.count(), series.mean(), series.std(),
   3775                            series.min(), series.quantile(lb), series.median(),
   3776                            series.quantile(ub), series.max()])

/home/wesm/code/pandas/pandas/core/series.pyc in f(self, axis, dtype, out, skipna, level)
    258         if level is not None:
    259             return self._agg_by_level(shortname, level=level, skipna=skipna)
--> 260         return nanop(self.values, skipna=skipna)
    261     f.__name__ = shortname
    262     return f

/home/wesm/code/pandas/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

/home/wesm/code/pandas/pandas/core/nanops.pyc in _nanmean(values, axis, skipna)
     77         np.putmask(values, mask, 0)
     78 
---> 79     the_sum = _ensure_numeric(values.sum(axis))
     80     count = _get_counts(mask, axis)
     81 

ValueError: could not find a matching type for add.reduce, requested type has type code 'M'
```
