added opencv as the optional dependency.

@pickle27 
I have added OpenCV as an optional dependency in the CMakeLists.txt.
OpenCV provides us with a OpenCVConfig.cmake file along with its installation. This helps us in a way that we no longer require to maintain a separate FindOpenCV.cmake file that we used to keep for other libraries in shogun/cmake/.

read the part after the heading "How package finding works" in the following wiki.
( http://www.cmake.org/Wiki/CMake:How_To_Find_Libraries,  )


As a practice, I tried finding if our other dependencies have gone super-responsible by providing us a Config.cmake file along with the installation over time. But sadly there none other than OpenCV for now.