Circle perimeter: add new method: Andres

This pull request is related to the discussion on the ML about hough transform for circles.

circle_perimeter was already in master. It implements the bresenham method. 
I discovered that this method as a drawback. Concentric circle does not fill entirely a disk as illustrated in figure 2b of http://hal.archives-ouvertes.fr/docs/00/58/05/56/PDF/FJT2006.pdf. Andres method does. For instance, it can be used to draw thick circles.

This patch contains andres method + tests. 
Bresenham is widely used, so it's the default. I decided to write a single function for both methods.

Observation:
image and image2 are identical.
```python
    from skimage.draw import circle 
    image = np.zeros((15, 15))
    image2 = np.zeros((15, 15))
    
    x_0, y_0 = (7, 7)
    for radius in range(0,7):
        x, y = circle_edge(y_0, x_0, radius, method='andres')
        image[y, x] = 1
    print(image)
    radius = 7
    x, y = circle(y_0, x_0, radius)
    image2[y, x] = 1
    print(image2)

```