Wu's anti-aliased circle + aa line + bezier curve + unittest

Hi,

This PR cancels this one: https://github.com/scikit-image/scikit-image/pull/542
It's basically Fedor's implementation + the missing unittest.

I also fixed the docstring.
We need to have a different returned tuple for Brenseham and wu algo. Is it ok like that?

I added Fedor in contributors. 

-----
Added later:
* Bezier curve
* AA line