Allow Panel to expand along major and minor indexes
related #4971

Currently Panel will allow us to expand along the _items_ dimension and will align cells in added DataFrames to the current _major_ and _minor_ indexes. However, if our DataFrame has a _major_ or _minor_ indexes not present in the original Panel, our append will silently fail (NaN s will populate the whole item)

I feel that a more natural behavior is for the Panel to expand along its major and minor axes to accomodate the new data.

I have to add here that I am really enjoying Panel and I find the logical progression from Series -> DataFrame -> Panel very well thought out.

Thanks.
