Add fallback decorator for 3D images

We don't support images greater than 2D, so fall back on ndimage
Should the same comment be repeated at every place that 
@default_fallback occurs?

On Sat 12 Jul 2014 08:18:28 PM PDT, Juan Nunez-Iglesias wrote:
> In skimage/morphology/binary.py:
>
> >
> >
> > +@default_fallback
>
> Actually, pretending I am a new reader here, this is really unclear.
> Could you add a few comments above this line, e.g.
>
> |# our functions don't work in 3D, so for 3D input we should fall back on `scipy.ndimage`.
> # Additionally, we want to use a cross-shaped structuring element of the appropriate dimension
> # for each of these functions. The `default_callback` provides all these.
> |
>
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> <https://github.com/scikit-image/scikit-image/pull/1063/files#r14856450>.
>