UTC localization / conversion broken in Timestamp
```
In [5]: ts_orig = Timestamp('3/11/2012 04:00', tz='utc')
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/wesm/code/pandas/<ipython-input-5-709530384ffe> in <module>()
----> 1 ts_orig = Timestamp('3/11/2012 04:00', tz='utc')

/home/wesm/code/pandas/pandas/lib.so in pandas.lib.Timestamp.__new__ (pandas/src/tseries.c:29685)()

/home/wesm/code/pandas/pandas/lib.so in pandas.lib.convert_to_tsobject (pandas/src/tseries.c:34455)()

AttributeError: 'NoneType' object has no attribute '_utcoffset'
```
