DatetimeIndex from timezone-aware datetimes fails
If you try to create a DatetimeIndex from a list of timezone aware datetimes, you get the exception: "Tz-aware datetime.datetime cannot be converted to datetime64 unless utc=True"

For example:

> > > import pandas, datetime, pytz
> > > d = [datetime.datetime(2012, 8, 19, tzinfo=pytz.timezone('US/Eastern'))]
> > > pandas.DatetimeIndex(d)
