Added CMake switch for lua or torch package paths

This allows one to build torch for use with luarocks without building torch itself as a rock. The cmake option WITH_LUA_PACKAGE_PATH needs to be set from the command-line to have an effect. This is because the actual full package paths are cached and should still be editable by the user when doing manual configuration using cmake-gui or ccmake.

The default is to use the existing torch package paths.