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
I don't think we should special-case the 8x8 matrix-multiplication. Instead, 8x8 should be the default value for `size` and it should just use the same code as above.