added example/matrix_multiplication.cpp

Hi,
Please review the example/matrix_multiplication.cpp

For some vague reason libboost_system was not getting detected, so added "system" in example/CMakeLists.txt
While the comments are nice, we don't really need doxygen-style markup for them in the examples (doxygen only gets run over the header files in `<boost/compute/...>`). Just a nice, human-readable description would be ideal.