DatetimeIndex.drop is losing timezone information

``` python

    def test_index_tz(self):
        ind = pd.date_range("2012-12-01", periods=10, tz="utc")
        ind = ind.drop(ind[-1])
        self.assertIsNone(ind.tz)
```

That seems counter-intuitive.
Freq information is also dropped, but that is IMHO less important

This is on pandas 0.10
