ENH: add expanding window time support
xref #16431 

[5] should easily work. This is just disabled ATM.

```
In [1]: dft = pd.DataFrame({'B': [0, 1, 2, np.nan, 4]},
   ...:    ....:                    index = pd.Index([pd.Timestamp('20130101 09:00:00'),
   ...:    ....:                                      pd.Timestamp('20130101 09:00:02'),
   ...:    ....:                                      pd.Timestamp('20130101 09:00:03'),
   ...:    ....:                                      pd.Timestamp('20130101 09:00:05'),
   ...:    ....:                                      pd.Timestamp('20130101 09:00:06')],
   ...:    ....:                                     name='foo'))
   ...:                                     

In [2]: dft
Out[2]: 
                       B
foo                     
2013-01-01 09:00:00  0.0
2013-01-01 09:00:02  1.0
2013-01-01 09:00:03  2.0
2013-01-01 09:00:05  NaN
2013-01-01 09:00:06  4.0

In [4]: dft.rolling('2s').sum()
Out[4]: 
                       B
foo                     
2013-01-01 09:00:00  0.0
2013-01-01 09:00:02  1.0
2013-01-01 09:00:03  3.0
2013-01-01 09:00:05  NaN
2013-01-01 09:00:06  4.0

In [5]: dft.expanding('2s').sum()
ValueError: min_periods must be an integer
```