export ravelin-config.cmake for developer custom install locations

[hsu]

for convenience, exporting custom ravelin-config.cmake to support custom install locations for developers.

One example might be gazebo-config.cmake.in and corresponding cmake install.

This way, if ravelin is installed outside of /usr/include and /usr/lib, it can still be found by

find_package(ravelin REQUIRED)
include_directories( ${RAVELIN_INCLUDE_DIRS})
link_directories(${RAVELIN_LIBRARY_DIRS})

etc.