Fasthog

This increases the speed to compute HOG features on an image. The histogram calculation has been moved to use cython. On the Lena test image, the computation time drops from 0.8 sec to 0.31 sec. This addresses issue #594.

```python
import skimage.color as col
import skimage.feature as feature
import skimage.data as data
import time

if __name__=="__main__":
	lena = data.lena()
	lenaG = col.rgb2grey(lena)

	st = time.clock()
	feature.hog(lenaG)
	print time.clock() - st
```