compat pickles don't have '_id'
So I'm running into an issue with creating a `Panel` out of legacy `DataFrame` pickles. The issue is that `_id` is set to `None` and the current `Index.is_` logic treats any two Indexes with unset `_id` as the same.

I thought the issue was with the unpickling. But it looks like there was an attempt to address this

``` python
def is_(self, other):
    return self._id is getattr(other, '_id', Ellipsis)
```

I'm guessing whoever thought that `getattr(other, '_id', Ellipsis)` would return `Ellipsis` if the `_id` was not set.

So I'm seeing:
1. Make unpickling make a new `_id` if it's not found in state
2. Make any `Index.is_` always return `False` if either one has an unset id.

Maybe both?
