ENH: Add unit keyword to Timestamp and to_datetime 

to enable passing of integers or floats that are in an epoch unit of s, ms, us, ns
    (e.g. unix timestamps or epoch s, with fracional seconds allowed)

closes #3540

```
In [5]: pd.to_datetime(Series([ 1370745748 + t for t in range(5) ]),unit='s')
Out[5]: 
0   2013-06-09 02:42:28
1   2013-06-09 02:42:29
2   2013-06-09 02:42:30
3   2013-06-09 02:42:31
4   2013-06-09 02:42:32
dtype: datetime64[ns]
```