Initial Implementation of FREAK descriptor

###TODO : 
- [ ] 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
```