BUG: Timestamps in object column not written as DateTime data in to_sql
When you have datetime data with timezone info attached, the data is stored as `Timestamp` objects in a `object` dtyped column, instead of in a ´datetime64´ column (as this cannot hold the timezone info). 
But, this causes `to_sql` to gives this the `TEXT` type:

With a postgresql engine:

```
In [48]: times = ['201412120154', '201412110254']
   ....: df = pd.DataFrame()
   ....: df['time'] = pd.to_datetime(times, utc=True)

In [49]: df
Out[49]:
    time
0   2014-12-12 01:54:00+00:00
1   2014-12-11 02:54:00+00:00

In [50]: df.dtypes
Out[50]:
time    object
dtype: object

In [51]: df.time.values
Out[51]:
array([Timestamp('2014-12-12 01:54:00+0000', tz='UTC'),
       Timestamp('2014-12-11 02:54:00+0000', tz='UTC')], dtype=object)

In [53]: print pd.io.sql.get_schema(df, 'test', con=engine_postgres)

CREATE TABLE test (
    time TEXT
)
```

This is because:

```
In [54]: pd.lib.infer_dtype(df['time'])
Out[54]: 
'datetime'
```

instead of `datetime64`, and this case is not handled at the moment.
