bool(MultiIndex) seems to always return False
This means that patterns such as:

``` python
if my_frame.index:
    # do something
```

with `my_frame` having a `MultiIndex` will never enter the `if` block.  One can check that `bool(my_frame.index)` returns False.  A single index (`pandas.Index`) will (correctly IMO) raise a `ValueError` warning that the truth value of an array is ambiguous.  However, a `MultiIndex` fails silently.

This is such an awesome source of silent bugs that I'm surprised this hasn't come up before.  Is there any reason why we want `bool` to return `False` every time for a `MultiIndex`?
