Implementing the FREAK descriptor

#### FREAK Paper link :
http://infoscience.epfl.ch/record/175537/files/2069.pdf

#### TODO : 
- [ ] Cythonizing some parts of the code
- [ ] Computing descriptor for keypoints on various scales
- [ ] Docs, tests

#### Current API :
```py
from skimage.data import lena
from skimage.color import rgb2gray
from skimage.feature import descriptor_freak
from skimage.feature import corner_harris, corner_peaks
img = rgb2gray(lena())
keypoints = corner_peaks(corner_harris(img), min_distance=1)
keypoints.shape
descriptors, filtered_keypoints = descriptor_freak(img, keypoints)
filtered_keypoints.shape
descriptors.shape
```