Timestamp.to_datetime fails with tzoffset objects
example from #2262

```
In [8]: df2['ts'][0].to_datetime() 
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-0663d65dd92c> in <module>()
----> 1 df2['ts'][0].to_datetime()

/home/wesm/code/pandas/pandas/tslib.so in pandas.tslib._Timestamp.to_datetime (pandas/tslib.c:8590)()

/home/wesm/code/pandas/pandas/tslib.so in pandas.tslib._Timestamp.to_datetime (pandas/tslib.c:8454)()

/home/wesm/code/pandas/pandas/tslib.so in pandas.tslib.convert_to_tsobject (pandas/tslib.c:10284)()

AttributeError: 'tzoffset' object has no attribute 'normalize'

In [9]: df2['ts'][0]
Out[9]: <Timestamp: 2012-11-15 10:38:04-0500, tz=tzoffset(None, -18000)>
```
