Adds cmake package configuration and version file to enable find_package(libpion).

This allows users of the library to include it easily :
       find_package(libpion @LIBPION_LIBRARY_VERSION@ REQUIRED)
       link_directories(${LIBPION_LIBRARY_DIRS})
       include_directories(${LIBPION_INCLUDE_DIRS})


       target_link_libraries(yourTarget ${LIBPION_LIBRARIES})