Integration of SEEDS superpixel algorithm

Hi

This pull request fully implements the SEEDS superpixel algorithm (http://www.mvdblive.org/seeds/) with C++ & Python samples and documentation.
I asked about this here: http://answers.opencv.org/question/36229/integration-of-seeds-superpixels-algorithm/

Please review it and provide feedback if something should be improved/added/...

Additionally I have a few questions:
- The algorithm uses (optionally) SSSE3. If enabled it runs quite a bit faster. I saw that it is disabled by default. Is there a possiblility to enable it by default & add runtime checks? The algorithm currently uses only the static `#if CV_SSSE3`.
- Does `fastMalloc` guarantee 16 byte alignment (on x86)? The algorithm relies on it as it uses SSE.
- What is the exact policy on changing interfaces?
   - What can be changed in a major/minor release?
   - What about adding methods, adding function parameters?
   - I ask because there are extensions planned, for example video.

Thanks
Beat