Blob Detection ( DoG ) - Resubmitting

Resubmitting #900

As @adonath has mentioned, the last pull request didn't provide significant speed improvements over the Laplacian Of Gaussians (LoG) method. This computes half the number of Gaussians the last method and is significantly faster than the LoG approach. But the trade-off is accuracy. The esitmated size of the blobs is less accurate than the previous method. LoG approach provides more accurate size estimation ( which I will implement later )

I haven't added an example here. I'll add it in my next PR after this is merged, which will include a example with the `Hubble Deep Field` image.

Examples
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![coin](https://f.cloud.github.com/assets/3823490/2323810/60b8ab8c-a3c1-11e3-9997-9869f29bb73d.png)
![space](https://f.cloud.github.com/assets/3823490/2323811/60c3c1fc-a3c1-11e3-907a-e3e6a044ef31.png)