FAST, oFAST and rBRIEF

### FAST Example :
```py
In [1]: from skimage.feature import corner_fast, corner_peaks

In [2]: from skimage.data import lena

In [3]: from skimage.color import rgb2gray

In [4]: img = rgb2gray(lena())

In [5]: corner_response = corner_fast(img, n=12, threshold=0.2)

In [6]: corners = corner_peaks(corner_response)

In [7]: corners.shape
Out[7]: (48, 2)

In [8]: from matplotlib import pyplot as plt

In [9]: plt.gray()

In [10]: plt.imshow(lena())
Out[10]: <matplotlib.image.AxesImage at 0x3bc2490>

In [11]: plt.scatter(corners[:, 1], corners[:, 0], s=50, facecolors='none', edgecolors='b')
Out[11]: <matplotlib.collections.PathCollection at 0x3bc2c50>

In [12]: plt.show()
```
![fast-lena](https://f.cloud.github.com/assets/2875129/1008576/81f33ad8-0b28-11e3-976b-dd6ef4be45c9.png)


### TO IMPLEMENT:
- [ ] oFAST
- [ ] rbRIEF