BRISK and FREAK added

 - BRISK detector and descriptor added (Issue #359).
 - FREAK Descriptor added (Issue #174)

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    img = Image("lenna")
    fs = img.findKeypoints(flavor="BRISK")
    fs.show()

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    img = Image("lenna")
    fs = img.findKeypoints(flavor="FREAK") # SIFT keypoints are detected. FREAK descriptor extracted.
    fs.show()

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    img = Image("lenna")
    fs, descriptor = img.getFREAKDescriptor("ORB") # ORB keypoints are detected. FREAK descriptor extracted.
    fs.show()

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FREAK descriptor for following keypoints can be computed

 - SURF
 - SIFT
 - BRISK
 - ORB
 - STAR
 - MSER
 - FAST
 - Dense