Fix plugin path

With this patch Jubatus try to load a plugin from JUBATUS_PLUGIN_PATH directory first, then load from the default plugin directory when a path name of the plugin doesn't contains '/', that means the path isn't relative nor absolute.
If it contains '/' character, Jubatus try to use `dlopen` directly. In this case, `dlopen` doesn't use LD_LIBRARY_PATH.
fix #695