ENH: Quantiles accepts an array
Doesn't quite finish #4196, but it should be easy not that `quantile` takes arrays.

I did this on top of #6953, so that should go in first.

Thanks, updated.
 Added a test on empty frames

Also fixed a bug, I wasn't passing the axis argument correctly when quantiles was an array.
 One more thing, thinking ahead to fixing #4196; Are we ok with returning a `FloatIndex`? When you do `df.describe()` the quantiles are formatted as (e.g.) `50%` instead of `.50`. I chose to use floats instead of formatting as strs since I see `quantile` being used in a pipeline, but `describe` is more useful as a final presentation thing.
 for `df,quantiles()` a `FloatIndex` is just fine (that's what your example was). I agree `describe` is more pretty output
 OK. Good to merge?
 can you add a test with datetimes both for Series and Frame for completeness?
 Do you mean with a DatetimeIndex or Datetime values? It doesn't look like we support quantile on `datetime` columns.

this is off master:

``` python
In [15]: df
Out[15]: 
           a  b
0 2010-01-01  1
1 2011-01-01  2
2 2012-01-01  3

[3 rows x 2 columns]

In [16]: df.quantile(.5)
Out[16]: 
b    2
dtype: float64

In [17]: df.set_index('a').quantile(.5)
Out[17]: 
b    2
dtype: float64
```
 have to pass `numeric_only=False`

I think it will blow up actually
datetime64[ns] need a view('i8') (and not cast to float
 iin docstring can you change ref from scipy.score..... to np.percentile  (as tht is what it uses now)
 yeah it blows up pretty badly. My simple fix didn't work. Separate issue?
 sure
 OK. I'll make an issue for that. This PR doesn't officially close any.
