ENH? to_datetime: ability to infer unit for numeric values
I implement a simple function for reference:

```python
def infer_as_datetime(dtarr):
    for unit in ['ns', 'us', 'ms', 's']: # for 'YYYY-MM-dd hh:mm:ss.msec' unit='ns' will succeed.
        dt = pd.to_datetime(dtarr[-3:], unit=unit) # tail values have higher chance with later time not in 1970
        if (dt.year > 1970).any(): return pd.to_datetime(dtarr, unit=unit)
    return pd.to_datetime(dtarr, unit='D')

print(infer_as_datetime([0,365,366]),infer_as_datetime(['1971-12-30 12:34:01.314','2015-01-30 22',]),infer_as_datetime([1442315569.315]),infer_as_datetime([1442315569000]),infer_as_datetime([1442315569000000]),infer_as_datetime([1442315569000000000]))
```
#### Expected Output

#### Output of ``infer_as_datetime(dtarr)``

<details>

DatetimeIndex(['1970-01-01', '1971-01-01', '1971-01-02'], dtype='datetime64[ns]', freq=None) DatetimeIndex(['1971-12-30 12:34:01.314000', '2015-01-30 22:00:00'], dtype='datetime64[ns]', freq=None) DatetimeIndex(['2015-09-15 11:12:49.315000'], dtype='datetime64[ns]', freq=None) DatetimeIndex(['2015-09-15 11:12:49'], dtype='datetime64[ns]', freq=None) DatetimeIndex(['2015-09-15 11:12:49'], dtype='datetime64[ns]', freq=None) DatetimeIndex(['2015-09-15 11:12:49'], dtype='datetime64[ns]', freq=None)
</details>
