BUG: DataFrame.quantile fails on datetime values
``` python
In [2]: df = DataFrame({'a': pd.to_datetime(['2010', '2011']), 'b': [0, 5]})

In [3]: df.quantile(.5, numeric_only=False)
Out[3]: 
a    1.278072e+18
b    1.278072e+18
dtype: float64
```

Series handles things fine.

We need to do a `view('i8')` on the date time array somewhere, but I had trouble just sticking it inside of the `f` that gets passed to `data.apply(f, ...)`
