BUG: Give correct ndim, shape and size properties
Treats it like 2d array rather than array of tuples. I like it because
it's more informative - but we could also just describe `.values`.

Fixes #4842.

Still working out some kinks.

Woah, this causes a segmentation fault? It must be because you can't override those properties without messing up numpy.

I'm going to push this to 0.14 when MI won't be a subclass of ndarray.
