reorganization of an OpenCV dependency

Hi Jochen,
as we discussed a few days before (closed pull request #733), I looked for a better place for leaving 'timers_opencv.hpp'. I suggest to put the file to the includes for the 'common' module where other timers are declared already. Since the include path of the common module is used almost everywhere there is no need to change any 'CMakeLists' files. What do you think?