Inconsistency in array_to_datetime "utc_convert"
If tslib.array_to_datetime sees a tz-aware datetime object and `utc_convert` isn't True, it raises.  But later a tz-aware datetime may be parsed from dateutil and it is ignored:

```
            elif PyDateTime_Check(val):
                seen_datetime = 1
                if val.tzinfo is not None:
                    if utc_convert:
                        _ts = convert_datetime_to_tsobject(val, None)
                        iresult[i] = _ts.value
                        try:
                            check_dts_bounds(&_ts.dts)
                        except ValueError:
                            if is_coerce:
                                iresult[i] = NPY_NAT
                                continue
                            raise
                    else:
                        raise ValueError('Tz-aware datetime.datetime cannot '
                                         'be converted to datetime64 unless '
                                         'utc=True')
[...]
                    try:
                        py_dt = parse_datetime_string(val, dayfirst=dayfirst,
                                                      yearfirst=yearfirst)
                    except Exception:
                        if _parse_today_now(val, &iresult[i]):
                            continue
                        if is_coerce:
                            iresult[i] = NPY_NAT
                            continue
                        raise TypeError("invalid string coercion to datetime")

                    try:
                        _ts = convert_datetime_to_tsobject(py_dt, None)
                        iresult[i] = _ts.value
                    except ValueError:
                        if is_coerce:
                            iresult[i] = NPY_NAT
                            continue
                        raise
```

There are several ways (some of which are hit in the tests) by which `parse_datetime_string` could return a tzaware datetime.  Should these raise iff utc_convert is not True?  Or behave like the string_to_dts parsing above and allow FixedOffset through?  What about if it is aware and already in UTC?