Blob Detection - Determinant of Hessian

The Determinant of Hessian approach implemented.

Computing the Hessian and then it's determinant was very slow, so I used an approximation with box filters over the integral image.

The method is different than the last two, because it is not specific to light blobs on a dark background


Examples
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![space](https://f.cloud.github.com/assets/3823490/2424638/709dda72-abaa-11e3-8f27-befc491f6809.png)
![sun](https://f.cloud.github.com/assets/3823490/2424639/70a21a1a-abaa-11e3-8ee5-e5cf2c6163da.png)

The `hessian_det_appx` might be useful elsewhere, should I make it public ?