Hough transform ellipse: review



* Use heapq for building the result. It will be much more easier and flexible for the end user.
* illustrate heapq in docs/ and doctests
* fix a "bug". The docstring claims that angle (from HT) was compatible with `ellipse_perimeter` but it was not. I fixed it and added a large quantity of tests (to convince myself :) ) The example looks cleaner (no weird np.pi - ... ).

This is the last PR for this feature, I hope :)