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. Someone's CMakeLists.txt could look as simple as this:

```
cmake_minimum_required(VERSION 2.8)
project(opensim-sample-client)

find_package(OpenSim REQUIRED)

if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU" OR
        "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
endif()

include_directories(${OpenSim_INCLUDE_DIRS})
link_directories(${OpenSim_LIB_DIR})

add_executable(myexe main.cpp)
target_link_libraries(myexe ${OpenSim_LIBRARIES})
```

This requires using CMake 2.8.6 (previously, we only depended on CMake 2.6).

Eventually, the API examples and the plugin examples can be cleaned up to use this mechanism.