feature branch : matrix-multiply-example

Hi,
Review
'matrix-multiply-example': adding the new multi-threaded
matrix-multiply example.

I think I am started to get this git stuff :)
I don't want to add `source.hpp` to `core.hpp` as it is not one of the core wrapper headers. Just add `#include <boost/compute/source.hpp>` here after `#include <boost/compute/core.hpp>`.