BUG: Constructing Timestamp from local time + offset + time zone yields wrong offset
xref #7825

``` python

In [1]: import pandas as pd

In [2]: pd.Timestamp('2013-11-01 00:00:00-0500', tz='America/Chicago')
Out[2]: Timestamp('2013-10-31 23:09:00-0551', tz='America/Chicago')
```

This bug is inherited from `datetime`, which is using local mean time (the first entry for `America/Chicago` in tzinfo).
