New manual chapter on vectorization

[Jordi Gutiérrez Hermoso]

I will push the cset soon, but this is how it's starting to turn out:

    
http://jordi.platinum.linux.pl/octave.html/Vectorization-and-Faster-Code-Execution.html

I have lots more to say about accumulation and vectorisation examples
as well as broadcasting, but as far as satisfying the release
requirements of 3.6, I think this close to done.

Suggestions welcome.

- Jordi G. H.