WIP: wrap ndimage morphology

Hey guys,

This should have been a simple patch but unfortunately the ndimage support for asymmetric structuring elements is rather lacking/weird... See [these lines](https://github.com/scipy/scipy/blob/ec20ababa400e39ac3ffc9148c01ef86d5349332/scipy/ndimage/morphology.py#L1285). At any rate I myself have no experience with these. For example, what is an opening with an asymmetric element? An erosion followed by a dilation with the same selem will not be a no-op for a simple pixel image... Which seems to be the expected behavior, based on the tests.

Any suggestions? @ahojnnes @emmanuelle @pratapvardhan?
Hm, I just see that there is a overflow, when the sum of the selem is greater than 255 now. Or am I missing something?