server: serve downloads from "/download/".
server: serve downloads from "/download/".

Also, refactor methods.

oblank/captcha,kaz8/go-captcha,zj8487/captcha,thomasmodeneis/captcha,rainyear/captcha,eleme/fork_captcha,dchest/captcha,msn217/captcha,ZiRo-/captcha,gotomypc/captcha,ZiRo-/captcha