Morph selem ellipse structuring element

Based on this [discussion](https://groups.google.com/forum/#!searchin/scikit-image/ellipse/scikit-image/TeZe6aVC1wY/cnaRzCjgosgJ) with @stefanv.

Added ellipse structuring element in skimage/morphology/selem.py and corresponding test case.
Add a one-line short description as the first line, e.g. "Generate ellipse-shaped structuring element\n\nLONG DESC"