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
some of these headers don't seem to be used/needed (like `wait_list.hpp` and `user_event.hpp` and `platform.hpp`). it's good to only include the headers that are used.