Provide OpenSimConfig.cmake file

Right now, there is no easy way for others to make C++ projects that use OpenSim.

This allows others to easily create client projects that use OpenSim. See https://github.com/chrisdembia/opensim-sample-project/blob/master/CMakeLists.txt for a sample OpenSim client project.

This requires using CMake 2.8.6 (previously, we only depended on CMake 2.6).