BUG: DatetimeIndex.insert doesnt preserve name and tz

`DatetimeIndex.insert` doesn't preserve `name` and `tz` attributes. Also modified `DatetimeIndex.asobject` to return an object `Index` which has the same name as original to cover the case when the result is being object `Index`. 

```
# normal Index preserves its name after insertion
idx = pd.Index([1, 2, 3], name='normal')
inserted = idx.insert(0, 1)
inserted.name
# normal

# But DatetimeIndex doesn't
dtidx = pd.DatetimeIndex([datetime.datetime(2011, 1, 3), datetime.datetime(2011, 1, 4)], freq='M', name='dtidx')
inserted = dtidx.insert(0, datetime.datetime(2011, 1, 5))
inserted.name
# None
```