replace unlicensed icons/images, add proper credits

replace unlicensed icons/images, add proper credits
  * fugue icons (http://p.yusukekamiyamane.com/) CC-BY-SA license
  * ornimental bookend (https://openclipart.org/detail/19869/ornimental-bookend---left-by-j4p4n) Public domain license
  * Paper_texture_v5_by_bashcorpo_w1000.jpg source found at http://bashcorpo.deviantart.com/art/Grungy-paper-texture-v-5-22966998 under Public Domain license
  * linecons (https://www.iconfinder.com/iconsets/linecons-free-vector-icons-pack) CC-BY license
  * add credits to COPYING, (get list of committers with git shortlog -sne, and previous contributors from the author's website)
  * Fixes https://github.com/shaarli/Shaarli/issues/10
  * Fixes https://github.com/sebsauvage/Shaarli/issues/8