Use copied images instead of symlinks in Japanese version

Thank you for this great ebook.

In Japanese version (https://github.com/astaxie/build-web-application-with-golang/pull/287), `ja/ebook/images` directory is a symlink for `../../ebook/images`.
This does not cause any problem for building ebook, but we can't see images on the Github web page through symlink.
I think it is Github's problem, but just copying directory like English version is useful to see on web for now.