BUG: construction of tz-naive and tz-aware should not coerce
```
In [1]: Series([Timestamp('20130101'), Timestamp('20130101', tz='US/Eastern')])
Out[1]: 
0   2012-12-31 19:00:00-05:00
1   2013-01-01 00:00:00-05:00
dtype: datetime64[ns, US/Eastern]
```

should instead be

```
In [2]: Series([Timestamp('20130101'), Timestamp('20130101', tz='US/Eastern')], dtype=object)
Out[2]: 
0          2013-01-01 00:00:00
1    2013-01-01 00:00:00-05:00
dtype: object
```