ENH: Timestamp/DTI. to epoch time
add a ``.to_epoch(unit='s')`` method to ``Timestamp`` and ``DatetimeIndex`` that returns the epoch for that unit. I think would default this to ``s`` as that seems pretty common, but allow any of our units.
```
In [19]: s = Series(pd.date_range('20160101',periods=3))

In [20]: s
Out[20]: 
0   2016-01-01
1   2016-01-02
2   2016-01-03
dtype: datetime64[ns]

In [21]: ((s-Timestamp(0)) / Timedelta('1s')).astype('i8')
Out[21]: 
0    1451606400
1    1451692800
2    1451779200
dtype: int64

```